N Korea to send thousands to help rebuild Russia's Kursk

North Korea will send thousands of workers to help rebuild Russia's war-torn Kursk region, Moscow's security chief has said.

Russian Security Council Secretary Sergei Shoigu, who held talks with North Korea's Kim Jong Un in its capital Pyongyang on Tuesday, described the deployment as "fraternal assistance", Russian state media reported.

Neighbouring South Korea and Japan were quick to condemn the plan, with Seoul it saying it was a violation of UN sanctions on North Korea.

For months concerns have swirled of deepening military collaboration between the two states, amid reports of thousands of North Korean soldiers helping Russia fight its war on Ukraine.

On Wednesday, Russia's TASS news agency quoted Shoigu as saying North Korea would send a "division of builders, two military brigades [of] 5,000 people", as well as 1,000 deminers to help with the "restoration" of the Kursk region.

"This is a kind of fraternal assistance from the Korean people and leader Kim Jong Un to our country," Shoigu was quoted as saying, according to an AFP report.

North Korean state media also added that the meeting saw both Kim and Shoigu discuss other "long-term plans".

South Korea was quick to respond, with a foreign ministry official saying they had "grave concerns" over the "continuing illegal co-operation between North Korea and Russia", local media reported.

Japan has also expressed worries over the co-operation.

"We are seriously concerned about these developments as it will worsen the Ukrainian situation and affect the regional security environment surrounding Japan," Chief Cabinet Secretary Yoshimasa Hayashi told reporters on Wednesday.

In November last year, Russia signed into law a mutual defence treaty with North Korea. Both countries had earlier said they would help each other in the event of "aggression" against either country, with Kim saying it took their relationship to a "new high level of alliance".

South Korean intelligence offficers say there are an estimated 15,000 North Koreans working in Russia under bilateral industrial co-operation programmes - a source of revenue for Pyongyang.

Separately, Western officials had in January told the BBC that at least 1,000 of an estimated 11,000 North Korean troops in Russia had been killed in just three months.

A Seoul lawmaker said in April that they believed there were about 4,700 North Korean casualties, including 600 deaths.

Analysts have said that Pyongyang could be paid, or may be given access to Russian military technology in exchange for the troops.

Both North Korea and Russia confirmed the presence of the North Korean troops in April - an admission that was condemned by South Korea and the US.

At the time, Seoul described the deployment as "illegal", saying that it was in violation of the UN Charter and the UN Security Council resolutions.

However, top Russian and North Korean officials have maintained regular contact as the war in Ukraine continues. When Shoigu visited Pyongyang earlier this month, Kim vowed to support Russia "unconditionally", including on "the Ukrainian issue", North Korean state media reported.

Earlier in April, Russia also claimed it had regained full control of the western Kursk region - which has been denied by Ukraine.