WILLIAM BRANGHAM: One day after visiting Washington, Ukrainian President Volodymyr Zelenskyy met today with Canadian Prime Minister Justin Trudeau.

And Ukraine claimed responsibility for another attack in Russian-occupied Crimea, hitting the headquarters of Russia's Black Sea naval fleet.

Today, Nick Schifrin sat down with Ukraine's national security adviser in Kyiv to discuss the possibility that Congress will not approve a vital aid package and the course of Ukraine's counteroffensive to free Russian-occupied territory.

NICK SCHIFRIN: Outside, Bakhmut Ukrainian soldiers advance through a moonscape.

They won this battle for what was once the village of Andriivka.

And in the last few days in Southern Ukraine, they pushed through some Russian defenses.

But the counteroffensive'S cost and challenge are great, says Ukraine's National Security and Defense Council Secretary, Oleksiy Danilov.

How do you think the counteroffensive is going?

OLEKSIY DANILOV, Ukrainian National Security and Defense Council Secretary (through translator): Difficult.

Hard.

But every day, we're inching forward.

What I mean is that, unfortunately, it is not only up to us when it comes to the fighting.

A lot depends on other factors, which, unfortunately, we have no control over.

JOE BIDEN, President of the United States: Mr. President, it's an honor to welcome you back to the White House.

NICK SCHIFRIN: That's a reference to Ukraine's partners, particularly in the United States.

President Volodymyr Zelenskyy received a warm welcome yesterday at the White House, but he also walked through a Congress fighting over the administration's next Ukraine aid package.

Danilov says the problem runs deeper.

OLEKSIY DANILOV (through translator): You see, the West has not come to a consensus on what would be Ukrainian victory.

They talk about assistance, but not about victory.

These are two different things.

If we do not have victory, then the very existence of our country and the war that our children and grandchildren would have to fight is only a matter of time.

That's why we need victory.

NICK SCHIFRIN: Yesterday, President Zelenskyy told senators: "If we do not get the next U.S. aid package, we will lose the war."

Can you explain that?

Why would you lose the war if you don't get the next U.S. aid package?

OLEKSIY DANILOV (through translator): Keep in mind that it is very painful for him to review reports that we receive every morning on how many of our boys and girls have died.

And if we had had the capabilities to end this war as quickly as possible, I mean, military capabilities, they would still be alive.

NICK SCHIFRIN: But what did he mean?

OLEKSIY DANILOV (through translator): It means we need help.

NICK SCHIFRIN: So far, the U.S. has sent weapons, vehicles and ammunition worth more than $46 billion.

Danilov suggested, if the American well runs dry, there's no backup.

What are you doing to prepare for the possibility of not having as many American weapons systems and ammunition as you have been using?

Is there a Plan B?

OLEKSIY DANILOV (through translator): We cannot say that we have some sort of separate discussions about some Plan B.

We have no desire to engage in some sidetrack discussions.

NICK SCHIFRIN: Yesterday, after an expanded meeting with Zelenskyy's team, President Biden went even further.

JOE BIDEN: I'm counting on the good judgment of the United States' Congress.

There's no alternative.

NICK SCHIFRIN: Danilov showed me an interactive map that tracks historic Russian airstrikes and today's front line in the most critical direction toward Crimea.

Are you trying to move farther down in this area to be able to threaten Russian supply lines?

OLEKSIY DANILOV (through translator): It's very simple.

We have to go to the sea.

NICK SCHIFRIN: Today, Ukrainian cruise missiles struck Russia's Black Sea Navy headquarters in Sevastopol, Crimea.

Ukraine appears to have increased its attacks and occupied Crimea, including on Russian warships last week.

OLEKSIY DANILOV (through translator): Well, starting from February 24, 2022, we're constantly trying to destroy the military potential in the temporarily occupied territory of our Crimea.

Several times, we have struck the so-called illegal Crimean Bridge, which is the military gateway for the Russian Federation.

We have achieved results in attacking Russian warships that fire missiles into our territory.

NICK SCHIFRIN: And Ukraine has also increased attacks deep inside Russia, including the Chkalovsky Air Base that hosts specialist military aircraft and VIP transport planes.

OLEKSIY DANILOV (through translator): These airfields house support planes from missile launch jets that fire rockets into our cities.

So why should we not destroy them?

We have been attacking them, and we will keep doing that until Putin withdraws all his troops from our territory.

NICK SCHIFRIN: As for the counteroffensive, the days ahead might be dark, but Danilov remains determined.

OLEKSIY DANILOV (through translator): Sooner or later, it will end, and I am more than sure that we will win.

And it will be a happy ending for our country.

It cannot be otherwise, because, if the darkness keeps winning, rest assured that, in that that case, our country will be first, and yours will be next.

NICK SCHIFRIN: National Security Adviser Danilov, thank you very much.