Ukrainian President Volodymyr Zelensky said North Korea is preparing to send 10,000 troops to help Russia’s war on Ukraine, in a sign of deepening military cooperation between Moscow and Pyongyang.

“We know about 10,000 soldiers of North Korea, that they are preparing to send to fight against us,” Zelensky told reporters in Brussels on Thursday. Ukraine officials have for weeks said that North Korean soldiers were fighting in Ukraine, without providing evidence.

U.S. Deputy Secretary of State Kurt Campbell said earlier this week that they were seeing signs of increased material support from North Korea to Russia, both artillery shells and missiles, though reports of personnel being sent were still being evaluated.

But speaking alongside Zelenskiy at another press conference, NATO Secretary General Mark Rutte said there was no evidence North Korean soldiers are involved in the fight.

Russia last week dismissed the reports of North Korean troop deployment as “one more fake.”

The deployment, if confirmed, would mark a major step in Pyongyang’s cooperation with Moscow after Russian President Vladimir Putin and North Korean leader Kim Jong Un agreed in June to provide immediate military assistance if one of them is attacked. North Korea maintains around 1.28 million active troops, according to South Korea’s defense white paper.

Washington and Seoul have accused Kim of sending artillery shells and ballistic missiles to Russia to aid the Kremlin, charges Pyongyang and Moscow deny.

In exchange for the arms, Russia has sent aid that has propped up North Korea’s economy and helped Kim advance his weapons programs, Seoul and Washington have said. 

Zelensky’s comments come as Western governments are struggling to maintain public backing for Kyiv’s fight against Russia, which has dragged on for more then two-and-a-half years. The wariness has prompted pressure on Zelenskiy to begin to factor in an endgame to the conflict.

Read More: Inside Ukraine’s Effort to Win Over Donald Trump

Zelensky presented his “victory plan” to lawmakers in Kyiv on Wednesday, outlining his terms for a potential end to Russia’s invasion. He reinforced his refusal to accept concessions on territorial gains or enter a process that would result in a frozen conflict.