AMNA NAWAZ: It's being described as the most serious intelligence leak in years.

Dozens of pages of classified material are spreading online after being originally posted on a gaming platform.

U.S. officials are investigating the origin of the leaks and the impact of information meant to be secret about the war in Ukraine, Russian forces, and even U.S. spying on American allies.

Nick Schifrin is here now to give us the details.

Nick, good to see you.

NICK SCHIFRIN: Thanks, Amna.

AMNA NAWAZ: Let's start with what we know.

What is in these documents, and how concerned are U.S. officials?

NICK SCHIFRIN: There is serious concern among U.S. officials about the U.S.' relationship with allies that are mentioned in these documents and about the front line in Ukraine.

So let's talk about Ukraine first.

We reviewed 53 of these documents dated from late February and early March mostly.

And we're not going to provide all the details they contain, but slides that seem to be prepared for the chairman of the Joint Chiefs of Staff reveal an extraordinary level of detail about Ukraine's coming counteroffensive, which Ukrainian brigades will be ready for the counteroffensive, when they will be ready, and with which weapons by date.

Many of those brigades going to be trained by the U.S., by the -- by allies on that vehicle there.

That is the Bradley.

And the level of detail is what is concerning U.S. officials.

It could give Russia information about these brigades that are being trained on the Bradley and others that Russia could convert into some kind of battlefield advantage, although there are former generals who actually doubt that that might happen.

The second thing that concerns officials are Ukrainian shortages.

Again, with great detail, these slides reveal exactly when Ukraine will run out of Soviet era air defenses.

That is the S-300 right there.

And that is why U.S. officials are rushing a constellation of Western air defenses you see there, U.S. German and Swedish air defenses.

And so, in response, Ukrainian officials have been largely vague about these leaks, as you would expect, but an adviser to Zelenskyy say: Look, our strategic decisions have already been made.

But we could adjust tactics if we need to.

AMNA NAWAZ: Well, what does the intelligence or what are these documents show about what the U.S. knows about Russian capabilities?

It doesn't exactly paint a great picture there, does it?

NICK SCHIFRIN: Not at all.

And the first thing that reveals is how much information the U.S. has about Russia war plans.

In fact, there's one detail that says that the U.S. shared with Ukraine details of an upcoming Russian offensive before Russia launched that attack.

Intercepted communications reveal especially about Bakhmut.

The private military contractor Wagner that you see there waging a scorched-earth campaign in Bakhmut, the U.S. intercepted communications within Bakhmut that reveal that Wagner was going to NATO ally Turkey to ask them for weapons and also offering Haiti some security assistance.

One last thing about Russia.

One slide reveals that Russian casualties killed and wounded could be 230,000 in this war.

We largely knew that, but some versions of that particular document online have a lower number than that, revealing that somebody, perhaps Russia, was doctoring these documents after they were leaked.

AMNA NAWAZ: You mentioned some information about the U.S. spying on our own allies, our own partners.

What do we see in these documents?

NICK SCHIFRIN: That the U.S. spies on some of its closest partners.

So let's take a look at some of the examples, one, Ukraine.

It definitely surveils Ukraine's leadership.

Two, the U.S. intercepted conversations within South Korea's National Security Council about whether it will send artillery to Ukraine and the U.S. intercepted communications with Mossad -- that is Israel's spy agency -- and that Israel has been asking the U.S. to conduct joint operations against Iran, something that, at least publicly, we did not know until now.

AMNA NAWAZ: Big question, Nick, what about the origin of the leak?

What do we know?

NICK SCHIFRIN: The U.S. officials who we speak to say they don't know much.

Take a listen to John Kirby, the spokesman of the National Security Council, earlier today answering a question to reporter about the leak seriousness.

JOHN KIRBY, NSC Coordinator For Strategic Communications: We don't know what's out there, James.

We don't know who's responsible for this.

And we don't know if they have more than they intend to post.

So we're watching this and monitoring it as best we can.

But the truth and the honest answer to your question is, we don't know.

And is that a matter of concern to us?

You're darn right it is.

NICK SCHIFRIN: But independent investigators believe they know how the leaks spread.

The organization Bellingcat investigates digital footprints, and they found the first leak on a small channel of the gaming platform Discord, says Aric Toler, Bellingcat's director of research and training.

ARIC TOLER, Director of Research and Training, Bellingcat: There's a bunch of mostly some teenagers and some other mostly young men who just like to play "Call of Duty" and "Halo" and stuff together.

The documents that have surfaced are, as a source I talk to said, just the tip of the iceberg.

There's a lot more that's out there.

I don't know exact how many out there, but I have heard the word hundreds put out there, that there are hundreds of other documents that were taken by the leaker and put on this small Discord server.

NICK SCHIFRIN: Investigators will also look at the documents themselves.

Take a look at this one.

They're actually photographs of documents.

On these papers, you can see the top there the level of classification, also an object apparently on the desk behind the document.

Investigators will look at that.

And, already, defense officials are looking at restricting some of the people who have access to this information.

AMNA NAWAZ: A lot more questions left to answer.

Nick Schifrin, thank you.

NICK SCHIFRIN: Thank you.

AMNA NAWAZ: So, how damaging could the release of these secret documents be on the war effort in Ukraine and on U.S. military cooperation with allies around the world?

For that, we turn to retired Lieutenant General Doug Lute.

He served on the National Security Council staff during both the George W. Bush and Obama administrations.

He also had extensive experience on the Joint Staff at the Pentagon, where many of these leaked documents originated.

General Lute, welcome back.

And thanks for joining us.

In your career, you have been in countless briefings where this kind of information has been handled coming from the intelligence arm of the Joint Staff.

Just how tightly held is that information typically?

How easy is it to print it or move it off the premises?

LT. GEN. DOUGLAS LUTE (RET.

), Former U.S.

Ambassador to NATO: Well, typically these sorts of documents would be produced by the intelligence staff inside the Pentagon and briefed to the chairman of the Joint Staff on a very routine basis, typically even five days a week, especially when there's an ongoing conflict, as there is in Ukraine.

It is exceptionally unusual that they would be disseminated officially in official channels beyond that.

So, this is a set of documents which appears to be those that are most tightly kept within the Pentagon itself.

AMNA NAWAZ: Does the fact that it is so tightly kept making it easier in some way to figure out the origin of the leak?

LT. GEN. DOUGLAS LUTE: Perhaps.

But I think it's too early to tell.

AMNA NAWAZ: I want to ask you about some of the contents there.

We heard Nick's reporting on the level of detail when it comes to Ukraine's capability and strategy.

What is your take on that?

Could that give Russian forces an advantage in the war?

LT. GEN. DOUGLAS LUTE: Well, I thought Nick outlined well the tactical impact, the tactical damage that's done by these -- these documents.

And, in particular, I'd be concerned about the level of detail for the units that are being prepared with new equipment, fresh training and so forth, being prepared for the upcoming Ukrainian offensive.

And if the Russians are able to piece together which units with which equipment, and eventually locations inside Ukraine, they might be able to detect, right, and essentially deduce, conclude the main attack, the main effort of the upcoming offensive.

That would be very tactically damaging.

AMNA NAWAZ: You also heard Nick's reporting about the degree to which the U.S. has been spying on its own allies and partners, information President Zelenskyy, conversations between U.S. and South Korean officials, intelligence reports about Israel Could those damage the relationship the U.S. has with those nations and intelligence-sharing efforts with them?

LT. GEN. DOUGLAS LUTE: I think most of our allies and partners understand that we collect on a broad range of intelligence targets, to include, in some cases, them.

I don't think that will come as a big surprise.

I think, however, the damage here is the loss of trust and confidence among allies and partners.

So, will the Ukrainians, for example, be reluctant or less revealing to us because there they fear that such documents could be leaked?

And what about others who are providing support to Ukraine?

Some of them don't wish their support to be made public.

So this is -- this lack of trust and confidence, this erosion among allies and partners, could be the big strategic impact of the leaks.

AMNA NAWAZ: General Lute, how worried are you there could be more sensitive information that U.S. officials have not yet detected published out there somewhere else?

LT. GEN. DOUGLAS LUTE: I think they simply don't know what they don't know at this -- at this point.

These documents were dated, as Nick said, some weeks ago.

So it's difficult to know whether there in the interim period there have been additional leaks.

So I think the investigation will just have to play itself out.

AMNA NAWAZ: What about the possibility of someone inside the Pentagon providing that information?

Is that a concern?

LT. GEN. DOUGLAS LUTE: Well, I think it's a concern because this is where the documents originated, but it's too soon to lay any blame or to point any fingers.

AMNA NAWAZ: That is retired Lieutenant General Doug Lute joining us tonight.

General Lute, always good to see you.

Thank you for your time.