A Russian strike on Kharkiv, Ukraine’s second-largest city, killed six civilians and injured 10 early Saturday, regional officials said.

Ukraine’s national police said the attack was launched by drones. It published pictures of blazes that had broken out on city streets and next to buildings.

“As of this morning, there are six dead and 10 wounded as a result of the night strike on Shevchenkivskyi district,” Kharkiv Mayor Ihor Terekhov said on the Telegram messaging app.

“The attack hit residential areas – at least nine high-rise buildings, three dormitories, a number of administrative buildings, a shop, a petrol station, a service station and cars were damaged,” he said.

The strike had occurred just after midnight, news reports said.

Kharkiv, in Ukraine’s northeast, has been a frequent Russian target, with attacks intensifying in recent weeks. On Wednesday, a drone attack on the city killed four people and badly damaged apartment blocks.

President Volodymyr Zelenskyy’s chief of staff, Andriy Yermak, told the news outlet Politico in an interview published on Thursday, that he saw Kharkiv as the most likely target for any new Russian offensive in May or June.

Kharkiv regional Governor Oleh Synehubov reported a strike on a residential building and a missile attack on a village north of Kharkiv city. No casualties were reported in either incident.

Air raid alerts remained in effect for Kharkiv and most of the country, including the capital Kyiv, for several hours after the strikes.