Yukiya Amano spoke as Japan scrambled to avert a meltdown at a stricken nuclear plant on Monday after a hydrogen explosion at one reactor and exposure of fuel rods at another, just days after the devastation that killed thousands.

“The Japanese authorities are working as hard as they can, under extremely difficult circumstances, to stabilise the nuclear power plants and ensure safety,” Amano said in a statement to member states of the International Atomic Energy Agency (IAEA) in Vienna.

“The nuclear plants have been shaken, flooded and cut off from electricity. Operators have suffered personal tragedies,” he said, according to a copy of his remarks. But “the reactor vessels have held and radioactive release is limited.”  Amano, a veteran Japanese diplomat, said Japan had asked the UN agency to provide expert missions.

“We are in discussions with Japan on the details,” he said.  The Fukushima nuclear complex, 240 km north of Tokyo, has already suffered explosions at two of its reactors on Saturday and on Monday, which sent a huge plume of smoke billowing above the plant.

The nuclear accident, the worst since the Chernobyl disaster in Ukraine in 1986, sparked criticism that authorities were ill-prepared.