Malfunctions have occurred in the cooling systems of the two units, Kyodo news agency said, citing a top state official. "Temperatures have been rising slightly. This requires attention," Chief Cabinet Secretary Yukio Edano told reporters, according to RIA Novosti.

Cooling systems of the Fukushima plant failed after the building was damaged following the massive earthquake last week, resulting in blasts in reactors number 1 and 3 Monday.

The reactor number 2 also overheated Monday, but the injection of water into the unit restarted it a few hours later, RIA Novosti reported.

Early Tuesday, a fourth reactor was hit by a blast, causing leakage of radioactive materials from the plant.

The Japanese transport ministry declared a no-fly zone within the range of 30 km from the blast-hit plant earlier in the day.