South Korea urged Russia to stop its "illegal cooperation" with Pyongyang and voiced "grave concern" on Friday as Moscow moved to ratify its defence treaty with North Korea.
Ukrainian President Volodymyr Zelensky warned shortly after that Russia was planning to send North Korean troops into battle against his country as early as Sunday, and urged world leaders to pile "tangible pressure" on Pyongyang.
Russian lawmakers on Thursday voted unanimously to ratify a defence treaty with North Korea that provides for "mutual assistance" if either party faces aggression. It will now be sent to the upper house of parliament, the Federation Council, for approval.
South Korea and the United States said that thousands of North Korean troops were training in Russia.
Ukraine said this week that North Korean soldiers had arrived in the "combat zone" in Russia's Kursk border region.
While stopping short of confirming it had put boots on the ground, North Korea said any troop deployment to Russia would be in line with international law.
"(Seoul) expresses grave concern over Russia's ratification of the Russia-North Korea treaty amidst the ongoing deployment of North Korean troops to Russia," the South Korean foreign ministry said in a statement.
It added that the South Korean government "strongly urges the immediate withdrawal of North Korean troops and the cessation of illegal cooperation".
Seoul said it would work with allies to "take appropriate measures" over the move, and the country -- a major arms exporter -- has suggested it could revise longstanding policy barring it from sending weapons directly to Kyiv.
- 'Punish escalation' -
Zelensky, following a meeting with defence officials on Friday, said North Korean troops could be sent in to fight Ukrainian troops this weekend.
"According to intelligence reports, on 27-28 October, Russia will use the first North Korean military in combat zones," he said on social media.
"The actual involvement of North Korea in hostilities should be met not with a blind eye and confused comments but with tangible pressure on both Moscow and Pyongyang to comply with the UN Charter and to punish escalation," he added.
A senior official within the Ukrainian president's office said the North Korean troops could be deployed in battle either to the Russian region of Kursk or in eastern Ukraine.
Russian President Vladimir Putin said in an interview aired on Friday on state television that it was up to Moscow how it uses the treaty's clause on mutual military assistance.
"What action we take with this clause -- that's still under question. We are in touch with our North Korean friends," Putin said.
"I mean to say that it's our sovereign decision, whether we use something or not. Where, how, whether we need this, or (if) we, for example, only carry out some exercises, training, passing on some experience -- that's our business," he added.
Seoul and Washington have long claimed that the nuclear-armed North is sending major shipments of arms to Russia.
One of North Korea's United Nations representatives said at the UN General Assembly's First Committee on Disarmament and International Security that the country was sending neither weapons nor soldiers to help Moscow.
The allegations by South Korea and others are "nothing more than groundless rumours aimed at tarnishing the image of DPRK", Rim Mu Song said, referring to the North by its official name.
"It is yet another smear campaign devised by Ukraine" to get "more weaponry and financial support from the US and Western countries".
South Korea's representative flagged videos circulating online of North Korean soldiers in Russian uniforms speaking Korean, but Rim said they "again totally reject the allegation" of troop deployment.
On Friday a diplomatic official argued, however, that Pyongyang would be well within its rights to deploy soldiers on Russian soil.
"If there is such a thing that the world media is talking about, I think it will be an act conforming with the regulations of international law," said Kim Jong Gyu, vice foreign minister in charge of Russian Affairs.
- 'Provocation' -
South Korean President Yoon Suk Yeol has called the deployment a "provocation that threatens global security beyond the Korean Peninsula and Europe".
Yoon also said South Korea will "review" its stance on providing weapons to Ukraine in its war with Russia, which the country has long resisted.
Seoul has already sold billions of dollars of tanks, howitzers, attack aircraft and rocket launchers to Poland, a key ally of Kyiv.
In June, South Korea agreed to transfer the knowledge needed to build K2 tanks to Poland, which experts have said could be a key step towards production inside Ukraine.
North Korea has adopted a new national anthem, state media reported on Friday, another move that experts suspect will further leader Kim Jong Un's drive to define his country as entirely separate from, and in opposition to, the South.
North Korea amended its constitution to define the South as a "hostile" state and last week blew up roads and railways that once connected the two countries.