North Korea will stand by Russia until its "victory" in Ukraine, Pyongyang's foreign minister has said, as the United States warned thousands of North Korean troops could be sent to the conflict in the coming days.

North Korea's Choe Son Hui made the remark during her visit to Moscow, as Western sources suggest up to 10,000 North Korean troops are on the brink of entering the war on Russia's side.

She said that North Korea had no doubt in the "wise leadership" of President Vladimir Putin, who signed a mutual assistance pact with Pyongyang this summer and massively warmed ties with the reclusive state.

"We will always stand firmly by our Russian comrades until victory day," Ms Choe declared in Moscow after talks with her Russian counterpart, Sergei Lavrov.

She also vowed that North Korea will continue to develop its nuclear arsenal, with Pyongyang widely suspected of wanting nuclear technology from Russia in exchange for its military support in Ukraine.

Neither country has denied the troop deployment reports, which Ms Choe and Mr Lavrov did not mention in their statements after their talks.

Mr Lavrov did however laud "very close ties" between the two countries' "armies and special services".

Zelenskyy hints at Tomahawk missiles request from US

President Volodymyr Zelenskiyy on Thursday strongly suggested that Kyiv has requested supplies of long-range US Tomahawk missiles, as he made critical remarks about "confidential" information he said had been leaked.

Tomahawk missiles have a range of 2,500 kilometres, far greater than any missile Ukraine currently has in its arsenal.

Such a weapon's delivery would almost certainly be seen by Russia as an escalation in its war in Ukraine.

Zelenskyy travelled to the United States last month to pitch a "victory plan" to President Joe Biden, which he said could help pressure Moscow to negotiate an end to the war in good faith.

The Ukrainian leader has since said the plan envisages a "non-nuclear deterrence package" that would only be used if Moscow does not end its full-scale invasion and continues to escalate the conflict.

The New York Times cited a senior US official on Tuesday as saying that Zelenskyy had asked for Tomahawk missiles, something the official said was totally unfeasible.

Ukraine leader urges allies to stop 'watching'

But many of Kyiv's Western allies, he later said on Telegram, had not supplied the long-range weapons needed to strike them, before calling on [Ukraine's allies] to stop "watching" and take action to tackle the presence of North Korean troops.

"The first thousands of soldiers from North Korea are near the Ukrainian border. Ukrainians will be forced to defend themselves against them," he said. "And the world will watch again."

Zelenskyy said Ukraine had pinpointed every location where North Korean soldiers were posted in Russia.

"But instead of such necessary long-range capability, America watches, Britain watches, Germany watches…," he said.

"Everyone in the world who truly wants the Russian war against Ukraine not to expand … must not just watch. They must act.

The United States has been Ukraine's most important source of military assistance since Russia's February 2022 invasion, but also sought not to do anything that might prompt nuclear-armed Russia to respond harshly or widen the conflict.

In May, Mr Biden gave Ukraine permission to strike at targets inside Russia using American weapons, but only to defend Kharkiv.

The officials said that US policy banning the use of American-provided long-range missiles inside Russia had not changed.

Ukraine has developed its own long-range attack drones during the war and used them to strike targets well inside Russia.

South Korean Defence Minister Kim Yong-hyun said on Thursday that in addition to troops, North Korea had sent more than 1,000 missiles to Russia as well as millions of munitions.

It comes as North Korea also test-launched a suspected new long-range missile this week, which Kim Jong Un expressed "great satisfaction" with.

Experts said the missile test seemed to have been carried out to divert attention from mounting international criticism over the purported troop deployment to Russia.

ABC/wires