The head of the United Nations nuclear monitoring agency has praised Japan's cleanup work at the nuclear power plant crippled by an earthquake and tsunami in March.

Yukiya Amano, director general of the International Atomic Energy Agency, visited the Fukushima Daiichi plant Monday to assess progress in containing damage from the biggest nuclear crisis since Chernobyl 25 years ago.

Amano wore protective coveralls and a mask for a tour of the plant. while there, he inspected the extent of the damage to the reactor buildings and examined cleanup activities.

The twin natural disasters in March shut down the plant's cooling system, causing reactor cores to melt down and leak radiation into the surrounding atmosphere.

Officials of the plant's operator, Tokyo Electric Power Company, said last week they expect to be able to bring all six Fukushima reactors to a safe state, known as a cold shutdown, within six months. But they acknowledged that a full cleanup may take years.

The earthquake left more than 20,000 people dead or missing, and tens of thousands homeless. Radiation leaking from the Fukushima plant also forced thousands from their homes.