Japan's prime minister warned that radioactive levels had become "significantly" higher around an earthquake-stricken nuclear power plant on Tuesday after explosions at two reactors, and the French embassy said a low level radioactive wind could reach Tokyo in 10 hours.
Naoto Kan urged people within 30 km (18 miles) of the facility north of Tokyo to remain indoors, underscoring the dramatic worsening of Japan's nuclear crisis, the world's most serious since the Chernobyl disaster in Ukraine in 1986.
"There has been a fire at the No. 4 reactor and radiation levels in the surrounding area have heightened significantly. The possibility of further radioactive leakage is heightening," a grim-faced Kan said in an address to the nation.
"We are making every effort to prevent the leak from spreading. I know that people are very worried but I would like to ask you to act calmly."
The French embassy in the capital warned in an advisory that a low level of radioactive wind could reach Tokyo -- 240 km (150 miles) south of the plant -- in about 10 hours.
Winds over the facility are blowing slowly in a southwesterly direction that includes Tokyo but will shift westerly later on Tuesday, a weather official said.
The reactor operator asked the U.S. military for help, while Kyodo news agency said radiation levels nine times normal levels had been briefly detected in Kanagawa near Tokyo.
As concern about the crippling economic impact of the double disaster mounted, Japanese stocks plunged nearly 9.0 percent to their lowest level in nearly two years, compounding a drop of 7.6 percent the day before. The two-day fall has wiped more than $500 billion off the market.
There have been a total of four explosions at the plant since it was damaged in last Friday's massive earthquake and tsunami. The most recent were blasts at reactors No. 2 and No. 4 earlier on Tuesday.