"We don't know the reason" for the smoke, Hidehiko Nishiyama of the Nuclear Safety Agency said.
As a precaution, officials have evacuated a 12-mile (20-kilometer) radius around the plant and advised those up to 19 miles (30 kilometers) away to stay indoors to minimize exposure.
And for the first time, Edano suggested that those downwind of the plant, even those just outside the zone, should stay indoors with the windows shut tight.
Survivors, meanwhile, buried the tsunami dead in makeshift coffins, resorting to wrapping some bodies in blue tarps.
In Higashimatsushima, about 200 miles (320 kilometers) northeast of Tokyo, soldiers lowered bare plywood coffins into the ground, saluting each casket, as families watched from a distance. Two young girls wept inconsolably, their father hugging them tight.
"I hope their spirits will rest in peace here at this temporary place," said Katsuko Oguni, 42, a relative of the dead.
Hundreds of thousands remained homeless, squeezed into temporary shelters without heat, warm food or medicine and no idea what to call home after the colossal wave swallowed up cities along the coast.
Tomoko Hosaka in Tokyo, Tim Sullivan in Higashimatsushima, Mary Clare Jalonick in Washington and Foster Klug in Seoul, South Korea, contributed to this report.