North Korea is sending thousands of construction workers to help rebuild a war-torn Russian border region, a Russian official said on Tuesday, as the Kremlin boasted of new steps in the deepening partnership between the two countries.

Sergei K. Shoigu, a close aide to President Vladimir V. Putin, met Kim Jong-un, the North Korean leader, in Pyongyang on Tuesday.

Afterwards, he told reporters that Kim had agreed to send 5,000 construction workers and 1,000 sappers — combat engineers — to Russia’s Kursk region. That is where North Korean troops fought alongside Russian forces this past winter and spring to push Ukrainian soldiers out of several hundred square kilometres of Russian territory. The sappers, he said, will work on demining the region.

North Korea’s Korean Central News Agency did not mention sending labourers to Russia in a Wednesday report on the meeting between Kim and Shoigu. But it said Kim had decided to cooperate with Russia based on “a correct understanding of the current situation”. It also said that Kim and Putin had been exchanging letters for several weeks.

In a video of Shoigu’s arrival distributed by Russian state media, Kim can be seen embracing the Russian visitor and telling him that “our cooperation is deepening”.

As many as 15,000 North Korean workers are already employed in Russia, South Korean intelligence officials said in April. Their labour violates UN Security Council sanctions but is of mutual benefit for both Moscow and Pyongyang. The North Korean government earns much-needed foreign currency by claiming much of the workers’ salaries, while Russia gets an infusion of help at a time when its labor force has been depleted by the war in Ukraine.

South Korea has reported a sharp uptick in North Korean labourers in Russia since last year, when South Korean officials said there were roughly 4,000 North Korean construction workers in Russia, each earning about $800 a month.

The plan announced by Shoigu on Tuesday would deepen that arrangement in a symbolically important region 4,000 miles from North Korea: Kursk, where an estimated 14,000 North Korean troops fought on Russia’s side for months until expelling Ukrainian forces from the area in April.

Shoigu, who was Russia’s defence minister until last year and is now the secretary of Putin’s Security Council, described the 5,000 North Korean labourers to be sent to Kursk as two brigades of “military construction workers.” He said they would help fix electrical and communications lines, roads and buildings after the region is demined.

“This is a kind of fraternal aid of the Korean people and the leader Kim Jong-un to our country and to the Kursk region in particular,” Shoigu said. On Wednesday, South Korea’s government called for an immediate end to what it called “illegal cooperation” between North Korea and Russia, saying that Moscow’s use of North Korean workers violates UN sanctions.

It was not clear how Russia would compensate North Korea for the labour in Kursk. South Korean analysts say that Russia has provided North Korea not only with fuel and food, but also with military materials and technologies in return for support in the war against Ukraine.