A top US officer voiced cautious optimism Thursday that a meltdown at Japan's quake-hit nuclear plant could be averted and said the military stood ready to bolster its aid in the crisis.

Admiral Robert Willard, who is overseeing American military assistance after Japan's earthquake and tsunami, said 450 radiological and disaster specialists were awaiting orders to deploy as Japanese teams tried to cool fuel rods in reactors at the damaged Fukushima plant.

"I'm cautiously optimistic that we're progressing in that regard (cooling the reactors) based on the restoration of power and the efforts that they've made to add water (to the reactors)," Willard told reporters at the Pentagon in a teleconference.

"Every effort is being made," Willard said from Pacific Command headquarters in Hawaii.

His comments came after the International Atomic Energy Agency said the situation at the atomic plant had not worsened "significantly" over the past 24 hours but warned it would be premature to talk about a ray of hope.

Despite concerns in Washington that Japanese officials may have failed to disclose the full scale of the damage at Fukushima, Willard praised his Japanese counterparts for promptly sharing all relevant information.

"This is a very transparent, open line of communication between US military forces and Japanese military forces that are conducting this relief effort together," he said.

But given the fast-moving, "volatile" conditions in Japan, he said there were times when countries in the relief effort had to operate with "imperfect information."

To survey the Fukushima site, the US military is flying unmanned aircraft, including Global Hawks, "that are able to image the reactors, determine heat sources in the reactors and actually photograph the reactor area," he said.

"We're exchanging this information with our Japanese counterparts and together we're trying to amass as much accurate data on the status of the Fukushima series reactors as we possibly can."

US military planes and helicopters equipped with sensors were collecting information on radiation from Fukushima as well teams on the ground, he added.

Willard said he had offered Japanese authorities an array of possible assistance and that a nine-member assessment team from the US military was in Japan looking at sending in reinforcements for the crisis at Fukushima.

"I have requested a force of about 450 radiological and consequence management experts to be available to us. They are on prepare to deploy orders," he said.

The US government and military have declared an 80-kilometer (50-mile) evacuation zone around Fukushima, but the admiral said US forces are venturing into the no-go area as needed to provide support for Japanese authorities.

"We will make excursions in that area as necessary, recognizing that the plume that is of such concern is blowing out to sea the vast majority of the time," he said.

The Japanese government has told people living up to 10 kilometres (six miles) beyond a 20-kilometre exclusion zone around the crippled plant to stay indoors. More than 200,000 people have already been cleared from the zone.