Seoul'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 rumors," a North Korean representative said at a committee meeting during the UN General Assembly.

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 the meeting, held late Monday in New York.

Pyongyang and Moscow have been allies since North Korea's founding after World War II, and have drawn even closer since Russia's 2022 invasion of Ukraine, with Seoul and Washington long claiming that 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 the North.

After Seoul summoned Moscow's ambassador in 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.

"We have seen reports the DPRK has sent forces and is preparing to send additional soldiers to Ukraine to fight alongside Russia," Robert Wood, US ambassador to the United Nations, told the Security Council.

"If true, this marks a dangerous and highly concerning development and an obvious deepening of the DPRK-Russia military relationship," Wood said.

The United States and its allies have already voiced concern about North Korea providing weapons to Russia, which invaded Ukraine in February 2022.

- Urge immediate withdrawal -

The South Korean government "strongly condemned" North Korea on Tuesday, urging North Korean troops to be immediately withdrawn.

"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.

The government "urged the immediate withdrawal of North Korean troops and declared that if the current military alliance between North Korea and Russia continues, it will not stand idly by."

"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.

On Tuesday also, the South Korean news agency Yonhap reported that Seoul is considering sending a team of personnel to Ukraine to monitor North Korean troops being deployed, citing a government source.

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

Yonhap also reported that a pro-Russia Telegram account posted a photo showing the Russian and North Korean flags side by side on a Ukrainian battlefield.