The UN nuclear watchdog said on Sunday that there has been "some positive developments "in efforts to stabilize Japan's crippled nuclear power plant.

However, the overall situation at the Fukushima No.1 plant remains very serious, Graham Andrew, a senior official of the International Atomic Energy Agency (IAEA), told the press.

"Off-site electrical power has been connected to the local substation for Unit 2 today," Andrew announced, adding "work is continuing under difficult conditions to connect power from the substation to the reactor building.

He said seawater is still being injected into the reactor pressure vessels of Units 1, 2 and 3, but water injection is not needed for Unit 4 as the reactor is in outage.

A positive development is that cooling has been restored to the reactor pressure vessels in Units 5 and 6. The progress means that the reactors are in a safe mode, with cooling systems stable and under control, low temperature and pressure within the reactor.

White smoke or vapour from Unit 3 is still being observed, but it is less intense than on previous days. Spraying of the reactor building with water is in progress.

Andrew also said radiation levels in major Japanese cities have not changed significantly since Saturday and remain below those which are dangerous to human health.

The IAEA radiation monitoring team has taken measurements between Tokyo and locations up to 150 km from the Fukushima site. New data from the team indicated contamination on the ground at a location 50 to 70 km from the Fukushima site.

The IAEA is sending additional monitoring experts to Japan, Andrew said.