GEOFF BENNETT: Today, for the first time, the U.S. government confirmed that North Korean forces are in Russia to help fight Ukraine.

The U.S. said 3,000 troops have traveled so far, what it called a serious escalation.

It also reveals North Korea is expanding its alliance with Russia to take on the U.S. and its allies.

Nick Schifrin reports.

NICK SCHIFRIN: In Eastern Russia, marching to fight someone else's war.

Russian cell phone video apparently shows North Korean troops deploying to Russia, receiving Russian equipment, the first wave of what Ukraine and South Korea say will be 12,000 North Korean troops.

The U.S. says it's not clear how they will be used.

But they're believed to be special forces known as the Storm Corps, and if they join the battle, they may suffer the consequences, says National Security Council spokesman John Kirby.

JOHN KIRBY, NSC Coordinator For Strategic Communications: If they do deploy to fight against Ukraine, they're fair game.

They're fair targets.

NICK SCHIFRIN: Already, North Korea has shipped Russia more than a million artillery shells and ballistic missiles that Russia uses to strike Ukrainian residential neighborhoods and injure and kill Ukrainian civilians.

But North Korea's willingness to send soldiers who could die on Russia's behalf helps cement their alliance, says the Center for Strategic and International Studies' Victor Cha.

VICTOR CHA, Senior Adviser, Center for Strategic and International Studies: Sending troops is about the biggest symbol of an alliance commitment that one country can make to another.

So it really shows that North Korea is all in with the Russians in terms of this war in Ukraine, this war in Europe.

NICK SCHIFRIN: In June, Russian President Vladimir Putin visited Pyongyang to sign a treaty with the North's leader, Kim Jong-un, that included a mutual defense pact.

Today, the U.S.' largest concern is that Russia will help accelerate North Korea's intercontinental ballistic missile or nuclear programs or improve North Korea's submarines.

VICTOR CHA: So, historically, Russia has been reluctant to provide really high-end military technology to North Korea.

But with this deployment of troops, Kim can exact a higher price for what he's doing for Russia.

And this could take the form of ICBM technology, nuclear submarine technology, things that Kim has said he really needs to round out his modern nuclear weapons force that would pose a direct threat to the U.S. homeland.

NARRATOR: The Reds continue their relentless advance over the rugged country.

NICK SCHIFRIN: It's been 70 years since North Korean soldiers fought in war.

Their deployment to Russia will give them invaluable experience, says the Mansfield Foundation's Frank Jannuzi.

FRANK JANNUZI, President and CEO, Maureen and Mike Mansfield Foundation: Really, the North Korean army lacks realistic combat experience.

So deploying their special forces to Russia and potentially into Ukraine is going to give them invaluable experience in modern warfare, drones, interoperability, combined operations forces.

This is going to be a real game changer for the North Korean military as they try to build a more capable force.

NICK SCHIFRIN: Today's announcement coincides with Kim Jong-un's first ever publicized visit to an ICBM base.

In the past, Kim has threatened force if North Korea is threatened.

But, today, he's trying to create a new strategic axis, willing to use force as a fellow traveler with America's other global adversaries.

FRANK JANNUZI: The June 2024 strategic partnership between North Korea and Russia, we're now seeing Kim Jong-un define that as part of a global struggle against U.S. and U.S. interests by sending combat forces to Russia.

They really seem to be saying to the world, a threat to Russia is a threat to us.

NICK SCHIFRIN: Until then, Ukraine fears North Korean troops will join Russian forces to evict Ukraine from seized Russian territory within days.

For the "PBS News Hour," I'm Nick Schifrin.