WASHINGTON, Oct 17 (AFP) - The United States will hold Russia accountable for "war crimes", the White House said on Monday, hours after Russia attacked Ukrainian cities with drones during morning rush hour, killing at least four people in an apartment building in Kyiv.

Ukrainian President Volodymyr Zelenskiy, in his Monday evening video address, said there had been more attacks. "Right now, there is a new Russian drone attack. There are (drones) that have been shot down."

Interfax Ukraine news agency said Telegram users had reported blasts in the town of Fastiv just outside Kyiv, as well as in the southern port of Odesa.

Russian forces also targeted infrastructure across Ukraine in the second wave of air strikes in a week, like the first coming in the morning with people going to work and school.

Ukrainian soldiers fired into the air trying to shoot down the drones after blasts rocked the capital Kyiv on Monday morning. An anti-aircraft rocket could be seen streaking into the morning sky, followed by an explosion and orange flames, as residents raced for shelter.

U.S. President Joe Biden's press secretary Karine Jean-Pierre told reporters that the White House "strongly condemns Russia’s missile strikes today" and said the attack "continues to demonstrate (Russian President Vladimir) Putin’s brutality".

Mentioning a new, $725 million military aid package announced for Ukraine last Friday, she said: “We will continue to stand with the people of Ukraine for as long as it takes.

“... We will continue to impose costs on Russia, hold them accountable for its war crimes."

A pregnant woman was among four people killed in the attack on the residential building, Kyiv mayor Vitali Klitschko said. Ukraine's Interior Minister Denys Monastyrskyi said there had been deaths in other cities but did not provide a full toll.

Black smoke poured out of the windows of the Kyiv apartment building and emergency service workers toiled to douse flames.

"I have never been so afraid ... It is murder, it is simply murder," said Vitalii Dushevskiy, 29, a food delivery courier who rents an apartment in the building.

His flatmate, who gave his name only as Nazar, said they had tried to leave only to find the staircase "all gone".

Nearby, Elena Mazur, 52, was searching for her mother, who had managed to call her to say she was buried under rubble.

"She is not picking up the phone," Mazur said, hoping her mother been rescued and taken to hospital.

Ukraine said the attacks were carried out by Iran-made "suicide drones", which fly to their target and detonate. The United States, Britain and France agreed that Iran supplying drones to Russia would violate a U.N. Security Council resolution that endorsed the 2015 nuclear deal between Iran and six powers. Iran on Monday stuck to its denial that it is supplying the drones to Russia, while the Kremlin has not commented.