Ukrainian President Volodymyr Zelensky said the thousands of North Korean soldiers sent to reinforce Russian troops on the front lines in Ukraine would push the nearly three-year war beyond the borders of the warring parties.

Western leaders say North Korea has sent around 10,000 troops to support Russia's military operation and warn that its involvement in a European war could also unsettle relations in the Indo-Pacific region, including Japan and Australia.

Zelensky said on Tuesday that he had spoken to South Korean President Yoon Suk Yeol and told him that there were already 3,000 North Korean soldiers at military bases near the Ukrainian front and that he expected that deployment to increase to 12,000.

Pentagon spokesman Pat Ryder said Tuesday that a “relatively small number” of North Korean troops are currently in Russia's Kursk region, where Russian troops are fighting to repel a Ukrainian invasion, and a few thousand more are on their way that way.

South Korea, which has been in close contact with NATO, the United States and the European Union over recent developments, warned last week that it could send weapons to Ukraine in retaliation for the North's involvement.

“There is only one conclusion: this war is internationalized and goes beyond the borders of Ukraine and Russia,” Zelensky wrote on Telegram.

The Ukrainian president also said he and Yoon agreed to increase their countries' cooperation and share more intelligence information, as well as develop concrete responses to Pyongyang's involvement.

More US military support?

In Washington, White House national security adviser Jake Sullivan met with Zelensky's top adviser on Tuesday to discuss North Korean troops as well as an upcoming wave of U.S. arms shipments to Kiev to help Ukrainians protect their to improve energy infrastructure, The Associated Press reported, citing White House officials familiar with their private conversations.

Sullivan and Andriy Yermak, head of the Ukrainian president's office, shared concerns that North Korean troops could be stationed in Russia's Kursk region and what such a development could mean for the war.

The officials, who were not authorized to speak publicly, said during the two-hour meeting at the White House that Sullivan also briefed Yermak on President Joe Biden's plans to add additional artillery systems, ammunition, hundreds of armored vehicles and more to the United States ahead of him Bringing Ukraine out of office in January.

Sullivan told Yermak that the U.S. government plans to provide Ukraine with 500 additional Patriot and ARAAM missiles by year's end to bolster air defense, the officials said.

Later Tuesday, Biden said Ukraine should strike back if North Korean troops invaded the country.

“I’m worried about it,” Biden said when asked about the presence of North Korean troops in the Kursk region.

“If they invade Ukraine, yes,” he said when asked whether the Ukrainians should strike back.

Meanwhile, North Korea said its top diplomat was visiting Russia, another sign of deepening ties.

North Korean Foreign Minister Choe Son Hui arrived in Russia's Far East on Tuesday en route to Moscow, Russian state media said. Russian state news agencies said it was not clear who Choe would meet during her second visit in six weeks.

The Kremlin said Russian President Vladimir Putin had no plans to meet them.

What role North Korean troops could play remains unclear.

“The numbers suggest this is more than a symbolic effort, but the troops will likely play a supporting role and represent less than 1 percent of Russia's armed forces,” said a statement from the Center for Strategic and International Studies think tank (CSIS).

“Russia is desperate for additional manpower and this is one element of Russia’s efforts to fill the ranks without a second mobilization,” she added, noting that the presence could increase.

Cities in Ukraine are being bombed

Meanwhile, Russian drones, missiles and bombs struck Kiev and Kharkiv, Ukraine's largest cities, in overnight attacks, killing four people and wounding 15 in a sustained airstrike, authorities said on Tuesday.

Since the large-scale invasion of its neighboring country, Russia has bombed civilian areas in Ukraine almost daily, causing thousands of casualties.

In the eastern Donetsk region of Ukraine, the Russian army is also using all its might to combat frontline defenses. The Russian Defense Ministry claimed that Russian troops captured the town of Hirnyk in Donetsk and the villages of Katerynivka and Bohoyavlenka.