The warning comes amid signs of progress in efforts to avert a catastrophic meltdown in Fukushima's six nuclear reactors.
Engineers today managed to rig power cables to all six of Fukushima's reactors. They also started a water pump at one reactor to reverse overheating that has triggered the world's worst nuclear crisis since the Chornobyl disaster in 1986.
But workers were later evacuated from the complex after gray smoke rose from a building housing one of the most damaged reactors.
Tokyo Electric Power spokesman Kaoru Yoshida says the problem at Reactor No. 3 is under investigation.
"At around 3:55 p.m., we received reports that there was slightly grayish smoke coming out from the southeast roof of nuclear Reactor No. 3," he said. "Due to this, we have evacuated the workers and we are checking the situation."
Japan's National Police Agency says it fears the death toll from the quake and tsunami will exceed 18,000. More than 8,600 people have been confirmed dead, while more than 12,800 are missing.
The World Bank says it could take five years for Japan to rebuild from the triple disaster, costing the country as much as $235 billion.
compiled from agency reports