President Joe 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 on Wednesday, after Russia launched a sophisticated missile and drone attack on Kyiv.

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.

U.S., South Korean and Ukrainian intelligence assessments say up to 12,000 North Korean combat troops are being sent to the war. The bulk of those troops were expected to be deployed in Russia’s Kursk region where Ukrainian troops have seized a swathe of territory.

Russia’s early morning missile and drone attack was the first on Kyiv in 73 days. President Volodymyr Zelenskyy has said that Russia is intensifying its strikes, apparently in an effort to discourage Ukrainians from continuing the war, which is approaching its 1,000-day milestone.

Russia appears to be pressing its advantage as doubt swirls about how Washington might change policy on the war after Donald Trump takes office as U.S. president in January. The U.S. is the biggest provider of military help to Ukraine.

Trump has slammed the Biden administration for giving Kyiv tens of billions of dollars in aid and has promised to quickly end the conflict. Ukraine’s international backers fear that any rushed settlement would mostly benefit Russian President Vladimir Putin.