North Korean troops have returned to fighting alongside Russia, South Korea's spy agency said.It also said there appears to have been a deployment of fresh troops.Ukraine said in January that at least 3,800 North Koreans had been killed or wounded in the war.North Korean troops have returned to fighting alongside Russia, South Korea's spy agency said, following reports of earlier heavy losses."Following about a monthlong lull, North Korean troops were placed back in the frontline region of Kursk starting in the first week of February," South Korea's National Intelligence Service said in a note to the press seen by South Korean news agency Yonhap."It appears that there has been a deployment of additional troops, but their size is still being examined," the intelligence agency said.The note came after South Korean newspaper The JoongAng cited unnamed sources as saying that Russian cargo ships and military aircraft had transported between 1,000 and 3,000 additional North Korean troops sometime in January or February.Business Insider was unable to independently verify the claim.Last fall, Western and South Korean intelligence agencies said that Pyongyang had sent around 11,000-12,000 troops to fight in Kursk, the Russian region under partial occupation by Ukraine.Russia was estimated to be paying around $2,000 a month per soldier, though the soldiers themselves are unlikely to see much of that.Dmytro Ponomarenko, Ukraine's ambassador to South Korea, told Voice of America in November that Pyongyang would likely maintain a presence of up to 15,000 troops in the war, rotating soldiers every two to three months.He said this could mean 100,000 North Korean troops cycling in and out of combat within a year.Ukrainian President Volodymyr Zelenskyy said last month that at least 3,800 North Koreans had been killed or wounded in Kursk.The UK's Ministry of Defence also said earlier this month that North Korean units had been withdrawn from frontline positions, likely to rest and refit before being redeployed."This is almost certainly primarily due to heavy losses sustained during attacks against Ukrainian-held positions," it said.Pyongyang has sent some of its best units to Russia special forces including members of its elite 11th Corps, also known as the "Storm Corps," considered to be committed and hardened fighters.But the US said in December that North Korean troops were being sent on "hopeless" human wave assaults against Ukrainian positions, taking more than 1,000 casualties in the space of just one week.According to the Kyiv Independent, Russia has, in recent days, ramped up its attacks in Kursk. The region is considered a key negotiating chip in any coming peace talks between Ukraine and Russia.