North Korea has started sending troops to fight with Russia in Ukraine, South Korea's spy agency has said as Seoul warned of a "grave security threat".

The allegation comes a day after Ukrainian President Volodymyr Zelensky said he believed 10,000 North Korean soldiers could join the war, based on intelligence information.

South Korean President Yoon Suk Yeol called for a security meeting on Friday and said the international community must respond with "all available means".

According to the spy agency, 1,500 troops have already arrived in Russia - with anonymous sources telling South Korean media the final figure could be closer to 12,000.

This comes as evidence mounts that North Korea is supplying Russia with ammunition, as recently demonstrated by the recovery of a missile in Ukraine’s Poltava region.

Moscow and Pyongyang have also been deepening their cooperation in recent months. Last week, North Korean leader Kim Jong Un greeted Russian President Vladimir Putin on his birthday, calling him his "closest comrade".

Friday's security meeting was attended by key officials from South Korea's National Security Office, the Ministry of National Defence, and the National Intelligence Service, Yoon's office said.

"[The participants] decided not to ignore the situation and to jointly respond to it with the international community using all available means," it said.

The allegation from the National Intelligence Service comes days after Ukrainian military intelligence sources said that Russia's army is forming a unit of North Koreans.

The BBC has reached out to the NIS for comment.

Earlier this week, Putin introduced a bill to ratify a military pact he made with Kim, which pledges that Russia and North Korea will help each other in the event of "aggression" against either country.

South Korea's spy agency, the NIS, said North Korean troops are training in Russian bases in Vladivostok, Ussuriysk, Khabarovsk, and Vlagoveshensk.

This appears to confirm information from a military source in Russia’s Far East, who told BBC Russian this week that “a number of North Koreans have arrived” and were stationed in one of the military bases near Ussuriysk.

Seoul's spy agency also released aerial photographs of Ussuriysk and Khabarovsk, where they say hundreds of North Korean troops have gathered, and another photo of North Korea's Chongjin port, where a Russian ship was reportedly shown carrying North Korean soldiers.

The NIS said it found that since August, North Korea has sent 13,000 shipping containers carrying shells, missiles, and anti-armour rockets to Russia.

As many as eight million 122-mm and 152-mm shells have been supplied to Russia, it said.

However, some military experts believe the Russian military units will have difficulties incorporating North Korean troops into their frontlines.

Apart from the language barrier, the North Korean army has no recent experience of combat operations, they said.

“They could guard some sections of the Russian-Ukrainian border, which would free Russian units for fighting elsewhere,” said Valeriy Ryabykh, editor of the Ukrainian publication Defence Express.

“I would rule out the possibility that these units will immediately appear on the front line.”

Additional reporting by Jake Kwon and Hosu Lee in Seoul