The Obama administration is sending a squad of robots to Japan to help efforts to regain control over the Fukushima nuclear plant, it has emerged.

"A shipment is being readied," Peter Lyons, who oversees nuclear power in the department of energy, told a Senate committee. "The government of Japan is very, very interested in the capabilities that could be brought to bear from this country."

The news came amid growing fears about the status of the plant's No2 reactor, one of three which workers have been pumping water into in an effort to keep the fuel rods from melting down.

Richard Lahey, who worked as a safety research officer at General Electric when it installed the plant's units, said part of the molten core seemed to have sunk through the steel 'lower head' of the pressure vessel and on to the concrete floor below. Officials seemed to have "lost the race" to save the reactor, said Lahey, a professor at the Rensselaer Polytechnic Institute in the US, who based his conclusion on analysis of radiation levels.

The Japanese government said yesterday that it was considering nationalising the operator of the crippled power plant at the centre of the worst nuclear accident in the country's history, amid mounting criticism of its handling of the crisis.

"Our preparedness was not sufficient," government spokesman Yukio Edano said. He said that when the current crisis was over they would examine the accident closely and thoroughly review safety standards.

The prime minister, Naoto Kan, fought off criticism of his role, insisting to MPs that a state of "maximum alert" would be maintained until the power plant had been made safe.

According to Lyons, engineers were making progress in resolving the emergency at Fukushima. "Current information suggests the plants are in a slow recovery from the accident," he told senators in Washington. But he and Bill Borchardt, director of operations for the US nuclear regulatory commission, would not predict when the crisis might be over. "I really can't even hazard a guess on how long that will be," Borchardt said.

The advantage of deploying robots at Fukushima was underlined last week when two workers were exposed to high levels of radiation and burned. The workers were standing in pools of extremely radioactive water in a reactor turbine room without adequate protective gear.

Robots, with electronics built to withstand radiation, can work in areas of Fukushima where radiation levels would soon kill a human engineer.

They can also help experts get a view on damage to the reactor core. Lyons said the robots would be equipped with cameras as well as devices to measure radiation.

"They could go places where you certainly wouldn't send a person," he said.

The department of energy has developed a number of remotely operated robots designed to clear up radioactive waste from department of energy test weapons sites, Lyons said.

The earliest versions were developed in the wake of the Three Mile Island accident in 1979 when robots were sent in to get a view of the damaged reactor, and to suck up radioactive water and partially melted fuel.

In addition to the robots, which will be accompanied by trainers to instruct Japanese workers in their operation, the department of energy earlier sent nearly 40 people and almost 8 tonnes of equipment, including devices that measure radiation from aircraft.

Lyons said US flights were only going within 2.5 miles of the plant, because of the elevated radiation levels.

News that the Japanese government could take a majority stake in the Tokyo Electric Power company (Tepco) came as the government admitted that its nuclear safety standards had been insufficient to protect the plant against the 11 March tsunami. Authorities vowed to overhaul regulations once the radiation leak is under control.

The struggle to contain radiation at the complex has been hit by near-daily mistakes, including two workers being drenched with radioactive water on Tuesday despite wearing supposedly waterproof suits.

Doubts over the future of Tepco, the largest power company in Asia, has coincided with anger towards its president, Masataka Shimizu, who has not been seen in public for several days.

Tepco officials said Shimizu, 66, had been ill, but said he had resumed work directing emergency operations at the company's headquarters in Tokyo.

Shimizu has not appeared before the media since 13 March; for six days from 16 March, as his employees battled to prevent stricken reactors from going into full meltdown, he reportedly did not attend crisis meetings or visit Tepco's HQ.

On 15 March Shimizu was on the receiving end of an outburst from Kan, who said the firm had been too slow to inform him of an explosion at the plant. Reporters overheard Kan demanding of Shimizu and other Tepco executives: "What the hell is going on?"

Japan's government is likely to compile a first emergency budget of 2tn yen (£15.2bn) for disaster relief and will aim to submit it to parliament in April, the Nikkei newspaper reported this morning.