The US has said for the first time that it has seen evidence that North Korea has sent 3,000 troops to Russia for possible deployment in Ukraine, a move that could mark a significant escalation in Russia’s war against its neighbour.

US defence secretary Lloyd Austin said it would be “very, very serious” if the North Koreans were preparing to fight alongside Russia in Ukraine, as Kyiv has alleged. But he said it remained to be seen what they would be doing there.

“There is evidence that there are DPRK troops in Russia,” Austin told reporters on Wednesday, using North Korea’s formal name, the Democratic People’s Republic of Korea.

Speaking to reporters later on Wednesday White House spokesperson John Kirby said the US believes at least 3,000 North Korean troops are undergoing training in Russia.

The US determined the North Korean soldiers were transported by ship in early-to-mid October from North Korea’s Wonsan region to the eastern Russian city of Vladivostok before being taken to three military training sites in eastern Russia, said Kirby.

“If they do deploy to fight against Ukraine, they’re fair game,” he said. “They’re fair targets and the Ukrainian military will defend themselves against North Korean soldiers the same way they’re defending themselves against Russian soldiers.”

In Seoul, South Korean lawmakers said Pyongyang had promised to provide a total of about 10,000 troops, whose deployment was expected to be completed by December, the lawmakers told reporters after being briefed by South Korea’s national intelligence agency.

The figure of 3,000 is twice a previous estimate of numbers of North Korean troops already in Russia.

Park Sun-won, a member of a parliamentary intelligence committee, said after the briefing: “Signs of troops being trained inside North Korea were detected in September and October. It appears that the troops have now been dispersed to multiple training facilities in Russia and are adapting to the local environment.”

Austin said the alleged North Korean deployment could be further evidence that the Russian military was having problems with manpower, after huge numbers of casualties on both sides in what has become a war of attrition.

The Kremlin has previously dismissed Seoul’s claims about the North’s troop deployment as “fake news”, and a North Korean representative to the United Nations called it “groundless rumours” at a meeting in New York on Monday.

Moscow and Pyongyang have also denied weapons transfers, but they have pledged to boost military ties and signed a mutual defence treaty at a summit in June.

Last Friday South Korea’s national intelligence service said the North had sent 1,500 special forces personnel to Russia by ship and they were likely to be deployed for combat in the war in Ukraine after training and acclimatisation.

Ukraine’s president, Volodymyr Zelenskyy, has also accused Pyongyang of preparing to send 10,000 soldiers to Russia. On Tuesday he called on his allies to respond to evidence of North Korean involvement in Russia’s war.

Lee Seong-kweun, a lawmaker on the South Korean committee, said Pyongyang authorities had tried to keep news of the deployment from spreading. “There are also signs of North Korean authorities relocating and isolating those families [of the troops] in a certain place in order to effectively control them and thoroughly crack down on the rumours,” Lee said, citing the spy agency.

Lee also said the agency had confirmed that Russia had recruited a “large number” of interpreters for the North Korean soldiers, while training them in the use of military equipment such as drones.

“Russian instructors are assessing that the North Korean military has excellent physical attributes and morale but lacks understanding of modern warfare such as drone attacks,” he said. “Therefore there could be many casualties if they are deployed to the frontlines.”

US officials, speaking on condition of anonymity, say more than 600,000 Russian troops have been killed or wounded in the war in Ukraine.

Austin said the North Korean deployment could point to a shortage of Russian recruits. “This is an indication that he [Vladimir Putin] may be even in more trouble than most people realise,” he said.

On Tuesday the South’s presidential office urged an immediate withdrawal of the North’s troops from Russia, warning that it may consider supplying lethal weapons to Ukraine if military ties between them went too far.