Japan's US envoy insisted Saturday there was no evidence a stricken nuclear reactor had gone into full meltdown but acknowledged there had been a "partial melt" of a fuel rod at the quake-hit plant.
Radiation was detected leaking from the Fukushima plant after Friday's massive earthquake and subsequent tsunami, and an explosion there Saturday sent authorities scrambling to avert a major meltdown.
But a somber Ambassador Ichiro Fujisaki said: "We do not see evidence of that at this time."
"What our government has announced is, that it was a blow-up of the outer building," he told CNN.
"There was a partial melt of a fuel rod, melting of fuel rod. There was a part of that... but it was nothing like a whole reactor melting down," said Fujisaki, adding that he was being briefed hourly on the situation.
An explosion blew off the roof and walls of the structure around the reactor at Fukushima No. 1 atomic plant, about 250 kilometers (160 miles) northeast of Tokyo after a killer earthquake and tsunami flattened the region.
The fear has been that evaporating cooling liquid would expose the fuel rods to air, triggering a nuclear meltdown and major radiation leak.
Authorities say the blast did not rupture the container surrounding the reactor and that radiation levels had fallen afterwards.
With tensions soaring over the nuclear crisis, the ambassador tried to put a brave face on the trying times in Japan, expressing gratitude for the aid offered by more than 50 nations during "one of the biggest challenges in our history."
"We are working every minute, every second, in order to have the situation under control," he said.
Fujisaki said the number of households without power had dropped from more than six million on Friday to 2.5 million late Saturday.