The Russian-held Zaporizhzhia nuclear power plant in Ukraine has lost all external power supply and is relying on diesel generators, a last line of defense to prevent a meltdown from overheating reactor fuel, the U.N. atomic watchdog confirmed.

"This morning at around 5 a.m. local time Zaporizhzhia Nuclear Power Plant lost all off-site power when its last remaining 750-kilovolt line was disconnected, its only remaining backup 330-kilovolt line having been damaged a few days ago and under repair," the International Atomic Energy Agency said in a statement on Thursday.