North Korea has sent more soldiers to Russia and re-deployed others to the frontline in the western Kursk region, according to South Korea’s intelligence agency.
It said the exact number of extra troops sent was unknown, but military officials quoted in South Korean media have said they believed it was at least 1,000.
Neither Moscow nor Pyongyang have confirmed the new deployment.
North Korea is thought to have suffered heavy losses in the four months since its troops were first deployed to help Russia in its war with Ukraine.
In January, Western officials told the BBC they believed at least 1,000 of the 11,000 troops sent from North Korea had been killed in the past three months. North Korea and Russia did not comment.
The South Korean intelligence agency also said on Thursday that North Korean troops already in Russia had been re-deployed to the front line in Kursk at the start of February “after about a month of lull”.
Ukrainian special forces fighting in Kursk told the BBC in January that they had not seen any North Korean troops for weeks. Ukrainian President Volodymyr Zelensky later confirmed they had returned to the front line.
Earlier reports attributed to South Korean intelligence suggested that the North Korean soldiers were unprepared for modern warfare, and were especially vulnerable to Ukrainian drones.
However, defectors and other military experts have told the BBC these troops should not be underestimated.
Russian President Vladimir Putin launched a full-scale invasion of Ukraine in February 2022. The fighting has led to heavy losses on both sides.
Last August, Ukraine made a lightning thrust into the Russian Kursk region, taking border guards by surprise. Russia has been trying to drive them out ever since.
While Ukraine’s gains have since been steadily pushed back, partly due to the arrival of the North Koreans in Russia in October, it still retains hundreds of square kilometres of Russian territory and is inflicting huge losses on its enemy.
Russia’s military has not publicly revealed its battlefield casualties since September 2022, when it said 5,937 soldiers had been killed.
Zelensky has put Ukraine’s military casualties at 45,100 – but a number of military experts in both Ukraine and the West believe the losses are much higher.
Reports began emerging that North Korean soldiers had been deployed in October, following the deepening of bilateral ties between Putin and North Korean leader Kim Jong Un. This included the signing a security and defence treaty.