The setbacks aggravated public fears that authorities might not be able to contain the expanding nuclear crisis.
Japan's chief Cabinet secretary, Yukio Edano, said radioactive steam might have escaped from the containment unit of a second reactor at the Fukushima No.1 (Daiichi) plant 150 miles north of Tokyo. The announcement followed unsettling news that a midmorning surge in radiation had forced emergency workers to halt their efforts to try to avert a meltdown of three other reactors at the plant, work that included the crucial task of keeping water on the reactors' overheated cores.
Photos: Scenes of earthquake destruction
The burgeoning crisis has imposed a deepening isolation on the earthquake- and tsunami-battered country, with foreigners fleeing in growing numbers, rescue crews mindful of exit routes and international flights being diverted from the capital.
Another quake, centered off the coast near Tokyo and given a preliminary magnitude of 6, jolted the capital shortly after Edano's announcement, further fraying nerves.
In the country's north, tens of thousands of residents within about a 20-mile radius of the Fukushima plant were essentially trapped indoors for a second day Wednesday, urged again by authorities to avoid going out unless it was an emergency. That confinement coincided with growing hardship across the quake zone, where temperatures have dropped and snow fell overnight.
"Yesterday we ate a bit of rice and one egg," said Yoshiko Tsuzuki, 55, a homemaker waiting in a line outside a grocery store on the outskirts of the battered city of Sendai. "We're hungry. I want to buy water and anything to eat. We need everything."
It remained unclear why a nation renowned for its efficiency has been unable to marshal convoys of supply trucks into the disaster area, as China did after its 2008 earthquake. Though military vehicles were evident, few emergency supplies were seen on the major arteries from Tokyo into the hard-hit Tohuku region and other seriously affected areas.
Even in cities that lie well outside the quake zone, daily life was increasingly disrupted by rolling blackouts and the curtailment of Japan's much-vaunted transportation network, both of which will be key to restarting the engine of the world's third-largest economy.
Stock prices stabilized Wednesday after tumbling for two days, but there was deepening gloom over the long-term financial outlook after the worst earthquake in the country's recorded history, a concern even among people who have far more immediate and pressing fears.
"I'm worried in the long term about Japan's economy," said Yoshiko Konno, in her 60s, as she charged her cellphone at a community center in Sendai. "Just think of one example: oysters! Are Americans and Europeans going to want to import Japanese oysters if they think there is a danger of radioactive contamination?"
Five days later, the true scale of the disaster is still unknown.
At least 10,000 people are feared dead, a tally that is expected to take weeks to finalize. About half a million others have been displaced by quake and tsunami damage or the evacuation triggered by the emergency at Fukushima, a once-obscure nuclear plant that is now the focus of worldwide attention.
The cause of a blaze that erupted earlier Wednesday at the Unit 4 reactor — also the scene of a fire the day before — was not immediately known. The plant's operator, Tokyo Electric Power Co., known as Tepco, said radiation levels were too high for firefighters to get close.
Later, authorities said the blaze seemed to be subsiding on its own, as the one the previous day did. But hours later, public television broadcaster NHK showed live aerial video of a plume of white smoke rising from the reactor.
At the plant, desperate and improvisational measures have become the rule.
Tepco said it abandoned as impractical the idea of using a helicopter to douse a boiling storage pool filled with spent fuel rods. The spent rods are usually submerged in the pool next to the Unit 4 reactor, which was not operating when the quake struck.
Dozens of emergency workers were pulled from the plant after radiation soared to levels that could cause radiation sickness. It later subsided, and hours later the workers returned.