Tokyo, March 22 (DPA) The operator of a troubled nuclear power plant in northeastern Japan resumed efforts Tuesday to restore power at reactors a day after gray smoke from a reactor building halted their efforts, news reports said.

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 were to resume 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 Monday to restore power at the six-reactor plant, 250 km northeast of Tokyo. No explanation was offered by government officials.

Later it appeared to have stopped, but "what seems to be white hazy smoke" was still rising from the building, Kyodo News reported Tuesday morning.

Smoke was also seen at reactor 2, but the plant's operator said it was believed to be steam. Kyodo reported "white-steam like vapour" was still coming out from reactor 2 in the morning.

Two armoured vehicles were to be mobilized Monday to remove rubble at the plant, which has hampered the cooling operations.

Two German-made pumping vehicles are to join the efforts. The M52 Multi-Z, made by Putzmeister Holding GmbH can dump 150 tonnes of water per hour from a height of 50 metres.

An external power source became available 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 Monday 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.

"The fact that off-site power is close to being available for use by plant equipment is the first optimistic sign that things could be turning around," Bill Borchardt, head of NRC operations, said in a public meeting Monday of the commission in Washington.

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. Tokyo also told Fukushima to suspend shipments of raw milk.