South Korea's spy agency said Friday that Pyongyang sent a "large-scale" troop deployment to help its ally, claiming that 1,500 special forces were already training in Russia's Far East and ready to head soon for the frontlines of the Ukraine war.

"As for the so-called military cooperation with Russia, my delegation does not feel any need for comment on such groundless stereotyped rumours," a North Korean representative said during the UN General Assembly in New York.

Seoul's claims were "aimed at smearing the image of the DPRK and undermining the legitimate, friendly and cooperative relations between two sovereign states," the representative told a committee meeting on Monday.

Pyongyang and Moscow have drawn closer since Russia's 2022 invasion of Ukraine, with Seoul and Washington claiming that North Korean leader Kim Jong Un has been sending weapons for use in Ukraine.

North Korean state media have not commented on the purported troop deployment.

Russia has also not confirmed the troop deployment, but defended its military cooperation with North Korea.

After Seoul summoned Russia's ambassador to South Korea to complain, the envoy "stressed that cooperation between Russia and North Korea... is not directed against the interests of South Korea's security."

Neither NATO nor the United States have confirmed the deployment, but both have cast it as a potentially dangerous escalation in the long-running Ukraine conflict.

The United States and its allies have already voiced concern about North Korea providing weapons to Russia.

Kim's powerful sister released a statement later Tuesday, calling Ukraine and the South "bad dogs bred by the US", and again accusing Seoul of sending unmanned drones to Pyongyang.

"Seoul and Kyiv are exact counterparts in going about bagging and letting loose reckless remarks against nuclear weapons states at random without follow-up capability," Kim Yo Jong said.

South Korea's military initially denied sending drones, but has subsequently declined to comment, even after Pyongyang released images of what it claimed to be a drone sent from Seoul.

Kim Yo Jong added that Pyongyang's investigation into the case is ongoing and the "truth behind the despicable provocation" by the South will be "analysed further in detail".

- Urge immediate withdrawal -

South Korea "strongly condemned" Pyongyang on Tuesday, urging North Korean troops to be immediately withdrawn, warning it would "not stand idly by" if the military alliance between the North and Russia continues.

"North Korea, which has been providing large-scale military weapons to Russia, sending troops to Russia's illegal war of aggression in Ukraine is a significant security threat not only to our country but also to the international community," said the National Security Council.

"In response to the advancement of military cooperation between Russia and North Korea following the deployment of North Korean combat troops, the government will implement phased countermeasures," it added.

Also on Tuesday, the South Korean news agency Yonhap reported that Seoul is considering sending personnel to Ukraine to monitor North Korean troop deployments, citing a government source.

According to the source, the team, if deployed, will likely consist of military personnel from intelligence divisions to analyse North Korean battlefield strategies and participate in interrogating any captured prisoners of war.