Russian drones, missiles and bombs smashed into Kyiv and Kharkiv, Ukraine’s biggest cities, in nighttime attacks, killing four people and wounding 15 in a continuing aerial onslaught, authorities said Tuesday.

Russia has bombarded civilian areas of Ukraine almost daily since its full-scale invasion of its neighbor almost three years ago, causing thousands of casualties. The Russian army is also pushing hard against front-line defenses in the eastern Donetsk region of Ukraine.

In another worrying development for Kyiv officials, Western countries say North Korea has sent some 10,000 soldiers to help Russia’s military campaign.

Ukrainian President Volodymyr Zelenskyy was expected to speak about that new threat at a meeting Tuesday in Reykjavik with the leaders of Denmark, Iceland, Finland, Norway and Sweden.

A Russian aerial attack struck Kharkiv, in Ukraine’s northeast, at around 3 a.m., hitting a house and killing four people, regional Gov. Oleh Syniehubov said. Nearly 20 houses were damaged in the attack, he said.

Several hours earlier, Russia dropped a glide bomb on the landmark Derzhprom building in Kharkiv city center, injuring seven people, Mayor Ihor Terekhov said.

Derzhprom, also known as the Palace of Industry, is included in UNESCO’s World Heritage List as an example of modernist architecture.

Terekhov said that Russia has concentrated attacks on Kharkiv in recent days. He urged people not to ignore air raid warnings.

Authorities in Kyiv said debris from intercepted Russian drones fell on two city districts, injuring six people.

Ukraine has also used long-range drones to disrupt Russia’s war machine and embarrass the Kremlin by striking targets on Russian soil.

A special forces academy in the Russian province of Chechnya was hit by drones, causing a fire that was quickly extinguished, according to Chechen leader Ramzan Kadyrov, who is close ally of Russian President Vladimir Putin.

It was the first drone attack of the war on Chechnya, which lies about 800 kilometers (500 miles) east of Ukraine.

(AP)