Tokyo: The operator of a troubled nuclear power plant resumed efforts to restore power at reactors a day after gray smoke from a reactor building stalled their efforts in northeastern Japan.

Meanwhile, Tokyo Electric Power Co. (TEPCO) said radioactive substances were detected in seawater near the Fukushima-1 nuclear power plant, which was damaged by the March 11 earthquake and tsunami.

Japanese military vehicles and Tokyo Fire Department trucks resumed their cooling operation, which involves showering water over overheating reactors 3 and 4 after their efforts were suspended due to the smoke.

The smoke seen pouring from the reactor 3 building forced workers to suspend operations on Monday to restore power at the six-reactor plant, 250 km northeast of Tokyo. No explanation was offered by government officials.

Smoke was also seen at reactor 2, but the plant's operator said it was believed to be steam. Two armoured vehicles were to be mobilised to remove rubble at the plant, which has hampered the cooling operations.

An external power source became available on late Monday for reactor 1, TEPCO said. The external power source is available for four reactors 1, 2, 5 and 6 out of the six.

But the reactors are still in a "tough" situation, said Industry Minister Banri Kaieda, while Prime Minister Naoto Kan told reporters the efforts to cope with the nuclear crisis were "moving forward slowly".

US Nuclear Regulatory Commission (NRC) officials said that conditions at Japan's Fukushima nuclear power plant seem to have improved, though frantic attempts to regain control over disaster-crippled reactors are continuing.

Meanwhile, TEPCO said radioactive iodine at levels 126.7 times higher than the legal limit and radioactive cesium 24.8 times higher were detected in seawater near the plant.

That was believed to have been caused by the nuclear accident, company officials said.

The government told four prefectures-Fukushima, Ibaraki, Tochigi, Gunma-to restrict shipments of spinach and "kakina", a leafy vegetable, as the detection of radioactive substances in the produce surpassed legal limits.