A missile attack on an apartment block in western Ukraine's Lviv killed four people on Thursday, in what its mayor said was the biggest attack on civilian infrastructure in the city since Russia's invasion of the country began.

While Russia regularly pounds Ukraine with missiles, artillery and drones, the Lviv region, hundreds of kilometres from the frontlines and near the Polish border, has largely been spared the aerial onslaughts.

"An apartment building was damaged as a result of the Russian missile attack," Ukraine's Interior Minister Igor Klymenko wrote on Telegram.

"The 3rd and 4th floors in two sections of the house were destroyed... As of 7:00 am (0400 GMT), 4 people were killed, 9 were injured."

Rescuers were working to reach those still trapped under rubble, he said.

More than 50 apartments had been "ruined" and a dormitory at Lviv Polytechnic University had been damaged, Mayor Andriy Sadovyi posted on Telegram.

"This is the biggest attack on Lviv's civilian infrastructure since the beginning" of Russia's invasion, he said.

"Consequences of the night attack by Russian terrorists," Ukrainian President Volodymyr Zelensky wrote alongside a Telegram video post showing a damaged building.

"There will definitely be a response to the enemy. A tangible one."

It was unclear how many missiles had been launched.

- 'Sorting through debris' -

Earlier, regional governor Maksym Kozytski said "several" missiles were "moving in the direction of the western regions", citing Ukraine's Air Force Command.

A separate video posted by Kozytski showed a multi-storey building with part of its top floor destroyed.

Emergency services were on the scene and rescuers were "sorting through the debris", he said.

"As of now, the rubble is being dismantled," he said. "We are doing everything possible to... save people."

Unverified videos posted to Telegram purporting to show the aftermath of a strike showed shattered glass littering the floor of what appeared to be a dormitory.

It was not clear if the videos showed the same strike.

On June 20, Lviv was hit by a major Russian drone assault on Kyiv and other cities.

Ukraine has recently bolstered its air defence systems with Western-supplied weapons and the number of Russian missiles and drones breaking through has diminished.

But the spokesman for Ukraine's air force, Yuriy Ignat, recently said that newly supplied systems were still insufficient to cover the whole country.

Slow weapons deliveries to Ukraine delayed Kyiv's planned counteroffensive, allowing Russia to bolster its defences in occupied areas, President Volodymyr Zelensky said in a TV interview broadcast Wednesday.

"Our slowed-down counteroffensive is happening due to certain difficulties in the battlefield. Everything is heavily mined there," he told CNN via a translator in the pre-taped interview.