Biden joined Ukrainian President Volodymyr Zelensky "in urging Russia to cease its military activities in the area and allow firefighters and emergency responders to access the site," a White House readout of a call between the two leaders said.

Russian troops attacked the Ukrainian nuclear plant -- Europe's largest -- early Friday local time, with images on a live feed showing blasts lighting up the night sky and sending up plumes of smoke.

A senior US official said the latest information "shows no indications of elevated levels of radiation, and we're monitoring closely."

US Energy Secretary Jennifer Granholm tweeted that she had spoken with her Ukrainian counterpart and said that the site's reactors are protected by "robust containment structures" and "are being safely shut down."

"Russian military operations near the plant are reckless and must cease," she said, adding that her department had activated its Nuclear Incident Response Team.

Russia's attack on the plant drew swift international outcry, with the UN's atomic watchdog warning of "severe danger" if the reactors were hit and British Prime Minister Boris Johnson accusing Russian President Vladimir Putin of endangering all of Europe.