Kyiv, Ukraine — Russia attacked the Ukrainian capital Kyiv on Wednesday with a sophisticated combination of missiles and drones for the first time in 73 days, authorities reported, as the Pentagon said most of the North Korean troops sent to help Moscow’s war effort were fighting to drive Ukraine’s army off Russian soil in the Kursk border region. South Korea’s spy agency also said North Korean troops were “engaging in combat” in the Kursk region, hours after the U.S. military shared its assessment of the growing Russian-North Korean cooperation.

Air raid warnings blared for hours as Russia targeted eight regions of Ukraine, firing six ballistic and cruise missiles and 90 drones, the Ukrainian air force said. Air defenses downed four missiles and 37 drones, and another 47 drones were stopped by electronic jamming, the statement said. 

The damage was being assessed.

The air assault came as most of the more than 10,000 North Korean troops sent by Pyongyang to help Moscow in the war were engaged in fighting in Kursk, where a Ukrainian army incursion three months ago has succeeded in holding a broad area of land, embarrassing the Kremlin.

Russia’s military has trained the North Korean soldiers in artillery, drone skills and basic infantry operations, including trench clearing, Pentagon deputy spokesperson Vedant Patel told a briefing Tuesday. The cooperation faces challenges, according to the Pentagon, including how to achieve military interoperability and overcoming the language barrier.

NATO Secretary-General Mark Rutte said previously that the deployment of North Korean troops to Russia was a sign of Russian President Vladimir Putin’s “growing desperation” as the full-scale war he launched against Ukraine nears the end of its third year, but he also said it marked “a significant escalation” in North Korea’s involvement in the conflict, and “a dangerous expansion of Russia’s war.”

Officials in Kyiv have said Russia deployed around 50,000 troops to Kursk in a bid to dislodge the Ukrainians.

Russia has in recent months been assembling forces for a counteroffensive in Kursk, according to the Institute for the Study of War think tank, though the timescale of the operation is not known.

Russia’s plans in Kursk and Ukraine’s efforts to hold back a Russian onslaught in the eastern Donetsk region coincide with political uncertainty over how the incoming U.S. administration of President-elect Donald Trump will change Washington’s policy on the war. U.S. military aid is vital for Ukraine, but Trump has signaled he doesn’t want to keep giving tens of billions of dollars to Kyiv.

President Biden intends to bolster U.S. military support to Ukraine in the final months of his administration, U.S. Secretary of State Antony Blinken said Wednesday.

The U.S. will “continue to shore up everything we’re doing for Ukraine to make sure that it can effectively defend itself against this Russian aggression,” Blinken told reporters at NATO headquarters, before planned meetings with allied envoys and Ukrainian officials.

Blinken warned that North Korea’s decision to send its troops into combat operations alongside Russian forces “demands and will get a firm response.” He didn’t elaborate.