BRUSSELS (AP) — North Korea has sent about 10,000 troops to Russia to train and fight against Ukraine “over the next few weeks,” the Pentagon said Monday. Western leaders say the move will deepen the nearly three-year war that has rattled relations in the Indo-Pacific region.

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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 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 number to increase to 12,000.

READ MORE: North Korea's foreign minister visits Russia as its troops train for battle in Ukraine

On Tuesday, Pentagon spokesman Maj. Gen. Pat Ryder said 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 in that direction go.

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.

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 impending wave of weapons that the United States will deliver to Kiev to help Ukrainians fight the war To improve protection of their energy infrastructure, White House officials are 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 conflict.

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.

READ MORE: What would North Korean troops in Russia mean for the war with Ukraine?

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.

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

Tensions on the Korean peninsula are at their highest in years as North Korea continues to conduct a series of provocative weapons tests and South Korea and the United States expand military exercises.

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 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 on Tuesday that Russian troops had captured the Donetsk town of Hirnyk and the villages of Katerynivka and Bohoyavlenka.

Zelensky also spoke about the war at a meeting on Tuesday in Reykjavik with the heads of state and government of Denmark, Iceland, Finland, Norway and Sweden. He said a conference would begin in Canada on Wednesday that would address Russia's abduction of tens of thousands of children from the occupied territories of Ukraine.

A Russian airstrike hit Kharkiv in northeastern Ukraine around 3 a.m., hitting a house and killing four people, regional governor Oleh Syniehubov said. Nearly 20 houses were damaged in the attack, he said.

Several hours earlier, Russia dropped a glide bomb on the landmark Derzhprom building in central Kharkiv, wounding seven people, Mayor Ihor Terekhov said. Derzhprom, also known as the Palace of Industry, is currently included in the UNESCO World Heritage List.

Terekhov said Russia had concentrated attacks on Kharkiv in recent days. He urged people not to ignore air raid warnings.

According to Kiev authorities, debris from intercepted Russian drones fell on two city districts, injuring six people.

Ukraine has also used long-range drones to disrupt Russia's war machine and embarrass the Kremlin by attacking targets on Russian soil.

According to Chechen leader Ramzan Kadyrov, who is a close ally of Russian President Vladimir Putin, a special forces academy in Russia's Chechnya province was hit by drones, causing a fire that was quickly extinguished.

It was the first drone attack of the war on Chechnya, which lies about 800 kilometers east of Ukraine.

Associated Press writers Aamer Madhani and Lolita C. Baldor in Washington and Jill Lawless in London contributed to this report.