SEOUL—North Korea and Russia acknowledged for the first time the deployment of North Korean troops in the Ukraine conflict, a sign of the deepening military alliance and mutual-defense pact between Pyongyang and Moscow.

According to Ukrainian officials, North Korea dispatched around 12,000 soldiers to Russia last fall. Seoul officials reported that Pyongyang sent an additional 3,000 soldiers this year. Ukrainian and South Korean officials have said most of the North Korean troops were deployed in Kursk, the region in southern Russia that was partly occupied by Ukrainian troops last summer.

Until recent days, however, neither Russia nor North Korea had confirmed the presence of North Korean troops.

On Saturday, the Kremlin claimed it had regained control of the Kursk region. That day, during a video call with President Vladimir Putin , Gen.  Valery Gerasimov , chief of the general staff of Russia’s armed forces, praised North Korean fighters for helping defeat Ukrainian forces, acknowledging their presence for the first time.

In an announcement posted to the Kremlin website Monday morning, Putin praised the North Korean troops for their role in Kursk.

“We are confident that the relations of friendship, good neighborliness and cooperation between our countries, forged on the battlefield, will continue to develop successfully and dynamically in all directions,” he said.

On Monday, North Korean state media reported that leader Kim Jong Un had decided to deploy troops under a mutual-defense pact he signed with Putin last year.

North Korea’s Central Military Commission said its soldiers made an important contribution to “liberating” the Kursk region and demonstrated the country’s “firm militant friendship” with Russia.

Despite Moscow and Pyongyang’s claims of taking back Kursk, Ukraine said its troops were still conducting operations in Kursk, as well as another Russian region, Belgorod.

“We are maintaining our presence on Russian territory,” said Ukrainian President Volodymyr Zelensky in a video address Sunday evening.

A North Korean soldier who was captured by Ukrainians and held in Kyiv. Photo: Sasha Maslov for WSJ

Ukrainian officials had hoped the Russian territory they seized last August would give Kyiv leverage in peace talks. But Ukrainian forces largely retreated from Kursk in March, facing heavy drone attacks and an overwhelming number of enemy troops, including the North Korean soldiers.

Acknowledging the presence of North Korean troops allows Putin to claim that Russia was under attack from Ukraine and legitimize their involvement in postwar recovery efforts as well, said Hong Min, a senior research fellow at the Korea Institute for National Unification, a government-funded think tank.

For Kim, the public confirmation establishes guarantees for Russia to support Pyongyang in case of a future conflict and could mean the North Korean leader is prepared to send more troops to support Moscow, said Hong.

“The consistent emphasis on the ‘blood alliance’ from both sides guards against any future weakening of relations,” Hong said.

The Kim regime can also score a domestic propaganda victory by framing the deaths of his soldiers as heroic feats, he added. About a third of the North Korean troops were either killed or wounded in battle, according to Ukrainian officials.

North Korea didn’t say how many troops it had dispatched but admitted to casualties on Monday, promising a monument for the fallen soldiers in Pyongyang.

Cooperation between Russia and North Korea has grown rapidly since the two sides signed the mutual-defense agreement last June. Pyongyang has provided artillery shells, rockets and missiles used to attack Ukrainian forces. Meanwhile, North Korean troops have gained valuable battlefield experience .

Several high-level delegations have traveled between Moscow and Pyongyang in recent months. In March, the Kremlin said it was preparing to welcome Kim to Russia this year.

North Korea said in its Monday statement that it is “an honor to have an alliance with such a powerful state as the Russian Federation.”

Write to Dasl Yoon at dasl.yoon@wsj.com