Vladimir Putin
Putin said during a visit to a military hospital that Russia was going to intensify its attacks on Ukraine. AFP

Russian lawmakers on Wednesday voted unanimously to ratify a landmark mutual defence pact with North Korea, as Kyiv and the West say Pyongyang sent thousands of troops to fight for Russia against Ukraine.

The agreement formalises months of deepening security cooperation between the two nations, which were Communist allies throughout the Cold War.

North Korea has become one of the most vocal and important backers of Russia's full-scale offensive on Ukraine.

The West has long accused Pyongyang of supplying artillery shells and missiles to Moscow to fire on Ukraine.

The latest accusations, based on intelligence reports, that North Korea has deployed around 10,000 troops to Russia suggest even deeper involvement in the conflict and have triggered an outcry and warnings in Seoul, Kyiv and Western capitals.

Ahead of the vote, presidential official Andrei Rudenko addressed the house, saying Moscow's relations with Pyongyang have reached new heights.

Rudenko praised North Korea for being the "only country in the world to publicly support" Russia's 2014 annexation of Crimea and 2022 annexation of eastern Ukraine following Moscow's full-scale offensive.

"I believe this treaty is very timely," he told the house.

The vote came as Donald Trump claimed a victory in the US presidential election.

Russian President Vladimir Putin and North Korean leader Kim Jong Un signed the strategic partnership treaty in June, during the Kremlin chief's visit to Pyongyang.

They said it would obligate "mutual assistance in case of aggression", with Putin calling it at the time a "breakthrough document".

It also commits them to cooperate internationally to oppose Western sanctions and coordinate positions at the United Nations.

Putin visited Pyongyang this summer in his first trip to North Korea in 24 years.

Kim has called Putin his "closest comrade" while Foreign Minister Choe Son Hui said last week that North Korea would "stand firmly by our Russian comrades until victory day".

She called Moscow's offensive against Ukraine a "sacred struggle" and said Pyongyang believed in Putin's "wise leadership".

The treaty will now go back to Putin's desk for a final signature.

The ratification comes as North Korea faces growing international pressure not to send its troops into combat alongside Russia.

South Korea warned Tuesday that a substantial deployment was already near the combat zone, including in Russia's western Kursk region, where Kyiv has been mounting a cross-border offensive since the summer.

"More than 10,000 North Korean soldiers are currently in Russia, and we assess that a significant portion of them are deployed to frontline areas, including Kursk," Jeon Ha-gyu, a spokesman for the South Korean Defence Ministry, said Tuesday.

Asked directly about the reports last month, Putin did not deny that North Korean troops had been sent to Russia.

And several other Russian officials have deflected requests to comment on Western intelligence.

"The Kyiv regime is trying to do everything to involve Seoul" in the conflict, Kremlin spokesman Dmitry Peskov told reporters on Tuesday, refusing to comment on the substance of the allegations.

In exchange for sending troops, the West fears Russia is offering North Korea technological support that could advance Pyongyang's nuclear weapons programme.

The reclusive state fired a salvo of short-range ballistic missiles early Tuesday, Seoul's military said, its second launch in days.

But sending North Korean troops to fight against Ukraine would be a major escalation in the conflict, more than two and a half years after Moscow launched its full-scale offensive.

Russia has seized the momentum on the battlefield this year as it grinds through the eastern Donbas region, capturing a string of towns and villages as Ukraine struggles with manpower and ammunition shortages.