Underscoring grave concerns about the Fukushima plant some 240 km (150 miles) north of Tokyo, U.S. Secretary of State Hillary Clinton said the U.S. air force had delivered coolant to avert a rise in the temperature of the facility's nuclear rods.

Tokyo Electric Power Co said pressure inside a reactor at its Fukushima-Daiichi plant rose after the cooling system was knocked out by the earthquake, the largest on record in Japan.

Kyodo news agency quoted the company as saying that the radiation level was rising in the turbine building and the pressure had risen to 1.5 times the designed capacity.