North Korea appears to have sent more troops to Russia despite its soldiers suffering heavy casualties fighting on the front lines in Ukraine, South Korea’s main spy agency said Thursday.

Seoul’s National Intelligence Service (NIS) said it was trying to figure out how many additional troops North Korea deployed to Russia, according to a brief statement. 

The South Korean agency also assessed that earlier this month North Korean troops were redeployed at fronts in Russia’s Kursk region after a reported temporary withdrawal from the region. 

The U.S. first revealed in October that Pyongyang sent around 10,000 troops to train in Russia and be eventually utilized on the battlefield.

The deployment marked a new milestone in North Korean support for Russia’s war, as Pyongyang was already supplying Moscow with large shipments of ammunition, missiles and other weapons since the Kremlin invaded Ukraine in February 2022.

The troops were reportedly trained at military bases inside Russia before being sent to the Kursk region to fight alongside Kremlin forces attempting to regain territory ceded to Ukraine.

But the North Korean soldiers appeared to fare badly in the fight, with the White House in late December saying they were getting injured at a rapid rate: more than 1,000 killed or wounded in just one week. 

The losses forced the North Korean troops to pull back from the front lines in January, Ukrainian and U.S. officials said at the time. 

But earlier this month, Ukrainian President Volodymyr Zelensky said that Pyongyang’s troops were back on the front lines, an assessment backed up by the NIS.

“Following a month-long lull, North Korean troops were placed back in the frontline region of Kursk starting in the first week of February,” the spy agency said Thursday.

The announcement follows a report in South Korean newspaper JoongAng that said North Korea has sent up to 3,000 additional troops since January by ship and military cargo planes, as reported by The Associated Press.

Officials and experts have speculated that in exchange for the soldiers, North Korean leader Kim Jong Un is hoping to gain Russian technology to upgrade his nuclear weapons program, as well as a valuable international ally for the isolated nation. 

President Trump, who has vowed to end the war in Ukraine, once sought a diplomatic relationship with North Korea, but such talks collapsed during his first presidency. Since taking office for his second term, Trump has hinted at reopening the door to diplomacy with Kim.