A Tokyo Electric Power Co. spokesman said the latest explosion happened at 4 p.m. EST Tuesday at the No. 2 reactor of Fukushima No. 1 nuclear power plant, Britain's The Telegraph reported.
It is the third explosion at the plant in as many days.
"It's a scene from hell, absolutely nightmarish," said Patrick Fuller of the International Red Cross Federation from the northeastern coastal town of Otsuchi.
A cloud of radioactive dust billowed from the plant after the second explosion and government officials said it was "highly likely" the fuel rods had begun to melt despite efforts to cool them with sea water.
The International Atomic Energy Agency would not rule out the possibility of a disaster like the one in Chernobyl in 1986, the worst nuclear power plant disaster in history, but said it was "unlikely."
The Fukushima Daiichi power station is 150 miles north of Tokyo, one of the most populous cities in the world. Fear of a meltdown and the resultant radioactivity released into the atmosphere grew after Monday's explosion at the No. 3 reactor, The Telegraph said.
The blast injured 11 workers, releasing as much radiation in an hour as would be normal in six months. The explosion exposed as many as 160 people to high doses, with 22 received treatment for radiation poisoning.
Three executives from the utility that runs the crippled complex acknowledged pumps channeling seawater into one of the reactors had stopped temporarily, a major setback in efforts to cool the superheated core, the Los Angeles Times reported.
"We are trying to reopen the valve," a Tokyo Electric Power Co. official said. "The fuel rods are exposed. We are trying to get the pressure down and pump water into the pressure vessel again."