Russian troops have occupied Ukraine's largest nuclear power plant, with managers working at "gunpoint" after a fire caused by their attack was extinguished, according to Ukrainian nuclear officials.

Countries around the world swiftly condemned the episode, with the US embassy in Ukraine warning an attack on a nuclear plant was a "war crime" and the United Nations Security Council convening an emergency meeting, according to diplomats.

In a statement Friday morning local time, the State Nuclear Regulatory Inspectorate (SNRI) confirmed the Zaporizhzhia nuclear power plant in southeastern Ukraine was occupied by Russian military forces, but said officials remained in contact with plant management.

The power plant's six reactors remained intact, though the compartment auxiliary buildings for reactor unit 1 had been damaged, the SNRI said in its statement. Four of the remaining units were being cooled down while one unit is providing power, the statement said.

Petro Kotin, head of Ukraine's nuclear power operator Energoatom, later reported that management were operating at "gunpoint." He said on Telegram that Russian forces "entered the territory of the nuclear power plant, took control of the personnel and management of the nuclear power plant."

"Today there is no connection, the station management works at invaders' gunpoint," Kotin continued. As for the staff, they were admitted in the morning to perform their duties. We do not currently have a direct connection to the station. We get information from the sources at the station."

Kotin warned that although the reactors are safe, further attacks could lead to "disaster."

Ukraine's State Emergency Services (SES) had earlier confirmed several dozen firefighters had extinguished a blaze that had started in a training building outside the main reactor complex, following shelling from Russian military forces....