As emergency crews prepared Wednesday to return to Japan's tsunami-scarred reactors, U.N. nuclear agency chief Yukiya Amano spoke of a "very serious" situation and said he planned to leave for Tokyo within a day.
Asked if events were out of control, he answered: "It is difficult to say."
Another senior agency official suggested that the crippled Fukushima Dai-ichi nuclear plant would have been able to withstand Friday's huge earthquake had it not unleashed the giant tsunami that crippled the plant's cooling systems.
James Lyons, director of the International Atomic Energy Agency's safety of nuclear installations, said the plant had been upgraded to withstand "peak ground acceleration" that was less than what the structure had experienced when the 9.0-magnitude temblor hit. He offered no details.
It was unclear what Amano hoped to accomplish with his visit; he acknowledged he had no agenda and no schedule of whom he would be meeting with.
While saying he would likely stay in Tokyo, he added: "I will certainly have contact with those people who are working there who tackled the accident, and I will be able to have firsthand information."
His announcement in part appeared prompted by growing unease among the agency's 35 board member nations who have complained that information coming from Japan on the rapidly evolving nuclear disaster is too slow and lacking in detail.
Amano himself displayed some frustration with the information reaching his emergency operations team at IAEA headquarters in Vienna.
"There's a constant flow of information, but there are certainly grounds for improvement," he told reporters. He said that he planned to brief board members on what he learned as soon as possible after his return.
Amano spoke as emergency workers, who were forced to retreat from the stricken Japanese nuclear power plant when radiation levels soared, prepared to return after emissions dropped to safer levels.
The pullback cost precious time in the fight to prevent a nuclear meltdown, further escalating a crisis spawned by last week's devastating earthquake and tsunami that pulverized Japan's northeastern coast and likely killed more than 10,000 people.
It was unclear what happened in the overheating reactors after late morning, when the workers stopped pumping in sea water trying to cool their fuel rods. Officials gave only sparse information about the reactors.
But conditions at the plant appeared to be worsening. White steam-like clouds drifted up from one reactor which, the government said, likely emitted the burst of radiation that led to the workers' withdrawal. The plant's operator reported a fire at another reactor for the second time in two days.
Amano has been careful to avoid dramatizing the situation, with his comments decidedly low-key since Friday's earthquake and tsunami overwhelmed the Fukushima Dai-ichi's operating systems and set off the rapidly worsening nuclear crisis.
But in answering that it is "difficult to say" whether events were now out of control, he came the closest yet Wednesday to acknowledging that the best efforts of emergency teams, plant operators and government authorities may not prevent the disaster from escalating.
"It is not the time to say things are out of control," he said. "The operators are doing their maximum to restore the safety of the reactors."
Diplomats have told The Associated Press that some board members are critical of the agency for not shouldering more responsibility on the ground in Japan, leaving Amano arguing that his agency can only offer help if it is asked by Tokyo.
On Wednesday, he announced that two small IAEA teams would soon be leaving for Tokyo to help with nuclear safety and radiation protection.