Tokyo Electric Power Co. (TEPCO) warned Saturday of blackouts and power shortage in a wide area of Japan, not just in quake-hit areas, as electricity is in short supply after its power facilities were damaged by Friday's powerful 8.8 magnitude earthquake.

Japanese authorities making efforts to contain rising pressure in nuclear reactors damaged by the quake were forced to release radioactive steam from one plant on Saturday after evacuating tens of thousands of residents from the area.

The quake-hit nuclear power plant Fukushima No. 1, about 250 kilometers (160 miles) northeast of Tokyo, "may be experiencing nuclear meltdown", Kyodo and Jiji news reported Saturday.

Tokyo Electric Power Co also said fuel may have been damaged by falling water levels at the Daiichi facility, one of its two nuclear power plants in Fukushima, some 240 km (150 miles) north of Tokyo.

Japanese Prime Minister Naoto Kan visited a quake-stricken nuclear power plant in Fukushima Prefecture on Saturday and said that small amounts of radiation have been released from one of the reactors.

As he visited other areas in northeastern Japan affected by Friday's mega-quake by helicopter, the prime minister saw the full extent of the catastrophe the nation now faces.

"I realized the huge extent of the tsunami damage," Kan said at a press conference upon his return to Tokyo.

The prime minister also said that Saturday is a critical day for rescue teams to find survivors.

The 50,000 rescue personnel deployed to the hardest-hit regions, including Japan's Self Defense Force, will do their utmost to help those in need, he said.

Regarding the controlled release of a small amount of radioactive steam from Fukushima No. 1 nuclear plant in order to reduce mounting pressure that could lead to a meltdown, Chief Cabinet Secretary Yukio Edano said the steam is not expected to cause any immediate threat to people's health.

"We are taking every possible measure to prevent disastrous developments," Edano said, adding that public should remain alert as more aftershocks are highly likely.

A series of aftershocks, from 5-6.8 magnitude, hit off Japan's east coast on Saturday following the massive earthquake struck Japan Friday afternoon.