KYIV, Ukraine (AP) – The Biden administration said on Thursday some 8,000 North Korean troops are now in Russia near Ukraine’s border and are preparing to join the fight against Ukrainian troops in coming days—a potentially serious escalation of the nearly 3-year-old war.

If they go into Ukraine, it would be the first time a third country puts boots on the ground in the war. Other countries on both sides of the divide have sent military aid, including weapons and training: Iran has supplied Russia with drones, and Western nations have provided Ukraine with modern weapons and financial and humanitarian assistance.

The United States says some of those troops have already moved near Ukraine’s border in the Kursk region, where Moscow’s forces have struggled to push back a Ukrainian incursion.

“We’ve not yet seen these troops deploy into combat against Ukrainian forces but we would expect that to happen in the coming days,” U.S. Secretary of State Antony Blinken said at a news conference with Defense Secretary Lloyd Austin and their South Korean counterparts on Thursday.

Mr. Blinken said Russia has been training North Korean soldiers in artillery, drones and “basic infantry operations, including trench clearing, indicating that they fully intend to use these forces in front line operations.”

South Korea’s spy chief told lawmakers earlier this month that 3,000 North Korean troops were being trained to use equipment including drones before being sent to fight in Ukraine. Neither South Korean officials nor American officials have provided details about how they knew about the North Korean troops, and many questions surround the impact of North Korean participation.

What Do We Know About the North Korean Troops?

Reports of North Korean troops arriving in Russia’s Kursk region to help Russian troops fight off a Ukrainian border incursion began more than a week ago. Ukrainian President Volodymyr Zelenskyy told reporters that North Korean officers and technical personnel have already been spotted in Russian-occupied territories of Ukraine.

“I believe they sent officers first to assess the situation before deploying troops,” Mr. Zelenskyy said. He has cautioned that the participation of a third country could escalate the conflict into a world war.

This week, U.S. officials said they estimate there are about 10,000 North Korean troops in Russia. Seoul and its allies assess that the number has increased to 11,000, while Ukraine has put the figure higher, at up to 12,000.

Mr. Austin has said North Korean troops are wearing Russian uniforms and carrying Russian equipment as they advance toward Ukraine, in what he called a dangerous and destabilizing development.

Mr. Austin added that it would be a “very, very serious issue” if Pyongyang indeed did join the war on Russia’s side.

Ukraine’s UN Ambassador Sergiy Kyslytsya, speaking at an emergency meeting of the UN Security Council on Wednesday, said they expect as many as 4,500 North Korean troops to be at the border this week and to begin directly participating in combat operations against Ukrainian forces in November.

What Is Ukraine Doing?

Ukraine is preparing as though combating North Korea in its territory is inevitable.

An injection of 10,000 North Korean troops, which is what both Ukrainian and South Korean intelligence have claimed, “could significantly destabilize Ukraine’s defense there and greatly accelerate the advancement of Russian forces,” said Glib Voloskyi, an analyst from a Ukrainian think tank, Come Back Alive Initiatives Center.

Speaking to South Korea’s KBS channel Thursday, Mr. Zelenskyy said that psychologically, 3,000 or 10,000 soldiers from North Korea will not impact Ukraine, because that number is insufficient. But he said Mr. Putin is testing the reaction of the West.

“Whether it will be 3,000, 10,000, or even 100,000 will directly depend on the West’s reaction to the use of North Korean forces. If the response is weak, there will be more. And then we will no longer be talking about psychological impact, but about real impact.”

How Is the West Reacting?

Western leaders have described the prospect of North Korean troops fighting alongside Russian soldiers against Ukraine as an escalation. Mr. Austin said officials are discussing what to do about the deployment, which he said has the potential to broaden or lengthen the conflict in Ukraine.

It has become a key topic as U.S. and South Korean leaders met this week in Washington, fueling concerns that the presence of those soldiers will further destabilize the Asia-Pacific and broaden Moscow’s war on Ukraine.

South Korean Foreign Minister Cho Tae-yul on Thursday condemned the deployment “in the strongest possible terms” and called for an immediate withdrawal of the troops. North Korea’s belligerent actions not only places the European continent but also the Korean peninsula under threat and that Seoul agrees “to take necessary measures accordingly,” he said.

German and British officials have also weighed in.

“It’s a kind of escalation and it shows us a very important, a very important aspect. International conflicts are approaching very rapidly,” German Defense Minister Boris Pistorius said.

United Kingdom Defense Secretary John Healey said there was “not just a concern about the potential for an escalation of conflict in Europe. There is an indivisible link with security concerns in the Indo-Pacific as well.”

Why Does Russia Need North Korea?

North Korea and Russia, both in separate confrontations with the West, have deepened their military cooperation in the past two years. In June, they signed a defense deal requiring both countries to provide military assistance if the other is attacked.

For analysts, the introduction of troops would be a sign that the war is not going as Russia planned.

“I think Ukraine is wearing down the Russian army as we talk. You don’t get thousands of soldiers from North Korea if your war is going well,” said Justin Crump, a former British tank commander who heads Sibylline, a strategic advisory firm. “You don’t require them.”

North Korea has already sent over 13,000 containers of artillery, missiles and other conventional arms to Russia since August 2023 to replenish its dwindling weapons stockpiles. Those missiles are being actively used against Ukrainian targets, officials in Kyiv say.