North Korea has sent around 10,000 troops to eastern Russia, which will likely augment Russian forces near Ukraine "over the next several weeks," a US National Security Council official said Monday, as President Joe Biden called the deployment "very dangerous."

A NSC spokesperson made the remarks after North Atlantic Treaty Organization (NATO) Secretary General Mark Rutte confirmed that North Korean troops have been deployed to Russia's western front-line Kursk region -- a development feared to further escalate the protracted war in Ukraine.

"We believe that North Korea has sent around 10,000 total soldiers to train in eastern Russia that will probably augment Russian forces near Ukraine over the next several weeks," the spokesperson said in response to a question from Yonhap News Agency. "A portion of those soldiers have already moved closer to Ukraine."

Earlier, Pentagon's Deputy Press Secretary Sabrina Singh made the same statement during a press briefing, according to a transcript posted on the Department of Defense's website.

"We are increasingly concerned that Russia intends to use these soldiers in combat or to support combat operations against Ukrainian forces in Russia's Kursk Oblast near the border with Ukraine," she said.

"Should DPRK soldiers be used on the battlefield, this would mark a further escalation and highlights President Putin's increasing desperation as Russia has suffered extraordinary casualties on the battlefield, and an indication that Putin may be in more trouble than people realize," she added.

DPRK stands for the North's official name, the Democratic People's Republic of Korea.

Singh warned that should North Korean troops enter into combat, there would not be limitations on the use of US-provided weapons on those forces.

"If we see DPRK troops moving in and towards the front lines, I mean, they are co-belligerents in the war. So they are fighting on these front lines and the Ukrainians are defending their sovereign territory and pushing the Russians back," she said. "So, this is a calculation that North Korea has to make."

Casting an early ballot at the state of Delaware Department of Elections, Biden made his first public comment on the North's troop deployment.

"(It's) very dangerous, very dangerous," he told reporters after casting a vote.

"The idea that Kamala's opponent's talking to Putin and discussing what should be done ... I mean, anyway," he added, apparently referring to former President Donald Trump.

South Korea's Defense Minister Kim Yong-hyun and US Defense Secretary Lloyd Austin plan to meet at the Pentagon on Wednesday for their annual defense talks. The following day, they will join Foreign Minister Cho Tae-yul and Secretary of State Antony Blinken for their "two-plus-two" meeting.

North Korea's troop deployment as well as the allies' measures to address it are expected to figure prominently in those two meetings.

In a press briefing, Matthew Miller, the State Department's spokesperson, also confirmed the deployment of around 10,000 North Korean troops to Russia, while voicing concern over Russia's apparent intention to utilize those personnel.

Miller said that at the upcoming two-plus-two meeting this week, the two countries' top defense officials and diplomats are expected to discuss Pyongyang's expanding partnership with Moscow including the troop deployment, and a variety of North Korea's provocative actions over the past few months.

Asked to comment on the United States' diplomatic communication with China on the military ties between the North and Russia, Miller said that Washington has voiced concerns to China over "destabilizng" actions by Pyongyang and Moscow.

"I'll let them speak for themselves, but we have been making clear to China for some time that they have an influential voice in the region, and they should be concerned about steps that Russia has taken to undermine stability, they should be concerned about steps that North Korea has taken to undermine stability and security," he said.

Hong Jang-won, the first deputy director of South Korea's National Intelligence Service, confirmed that North Korea's deployment of units to Kursk is happening more rapidly than previously anticipated. He was in Brussels, leading a South Korean delegation tasked with briefing the transatlantic alliance on the North's troop dispatch.

"We (initially) reported to the government that it would take until early December, but it seems that both Russia and North Korea have accelerated their pace since the intelligence became public," Hong said.

During a phone call with European Commission President Ursula von der Leyen on Monday, President Yoon Suk Yeol described the current situation as "grave," noting that North Korean troops' actual entry into front lines could come sooner than expected.

Earlier this month, NIS said that North Korea was expected to send a total of around 10,000 troops by the end of this year, including some 3,000 troops already dispatched. (Yonhap)