The operator of the Fukushima Daiichi nuclear power plant said fuel rods in two more reactors were likely to have suffered a meltdown soon after they were crippled by the 11 March earthquake and tsunami in north-east Japan.

Confirmation by Tokyo Electric Power (Tepco) that fuel in the cores of reactors 2 and 3 had melted came days after new data confirmed a similar meltdown in reactor 1 about 16 hours after the disaster.

The utility, which last week suffered the biggest annual loss by any Japanese firm outside the financial sector, said most of the melted fuel in all three reactors was covered in water and did not threaten to compound the world's worst nuclear accident since Chernobyl.

The temperature of the fuel rods, which are believed to have melted and settled at the bottom of flooded reactor pressure vessels, remained well below dangerous levels, the company said.

"It is unlikely that the meltdowns will worsen the crisis because the melted fuel is covered in water," said a Tepco spokesman, Takeo Iwamoto.

It said the fuel rods in the reactors 2 and 3 had started melting two to three days after the earthquake and tsunami, which knocked out vital cooling systems.

Tepco officials repeated their insistence that the reactors had been crippled by the waves, but speculation has mounted in recent days that the quake itself had been responsible, casting doubt on Tepco's claims that the plant was able to withstand even the most violent seismic shifts.

Tepco said it had been unable to confirm the meltdowns until it had finished analysing data, but Koichi Nakano, a political science professor at Sophia University, suggested the revelation was timed to minimise its impact on the public.

"In the early stages of the crisis Tepco may have wanted to avoid panic," he told Reuters. "Now people are used to the situation … nothing is resolved, but normal business has resumed in places like Tokyo."

Tepco's handling of the crisis will come under closer scrutiny with the arrival in Tokyo of a delegation from the International Atomic Energy Agency (IAEA). The UN nuclear inspectors will visit the Fukushima plant and present their findings at a meeting of ministers from IAEA member-states on 20 June.

Tepco has suffered recent setbacks that may derail attempts to bring the plant under control in the next six to nine months, which is the deadline the firm announced just over a month ago.

On Monday, it said makeshift containers being used to store tens of thousands of tonnes of contaminated water were almost full.

The company has yet to complete a system to reprocess the water for reuse in the reactors, raising fears that contaminated liquid could leak into the sea.

A Tepco spokesman said dealing with contaminated water that has gathered in reactor buildings and trenches could take until the end of the year, adding that the volume of water being used to cool the damaged reactors could rise to about 200,000 tonnes.

Tepco is working with the French nuclear engineering firm Areva to reprocess the water.

Japan's shift towards renewable energy, meanwhile, is expected to gather momentum later this week.

The prime minister, Naoto Kan, will unveil plans at the G8 summit in Deauville, France, to require all new buildings to be fitted with solar panels by 2030, the Nikkei business newspaper said.

Kan has already announced a comprehensive review of Japan's nuclear energy policy and ordered the temporary closure of an atomic plant in central Japan that is considered particularly vulnerable to earthquake damage.

But he is also expected to tell G8 leaders that Japan will continue to use nuclear energy after making safety improvements.

Some have criticised Kan and Tepco for failing to quickly release information about the extent of the damage at Fukushima Daiichi. "I am very sorry that the public doesn't trust the various disclosures the government has made about the accident," Kan told parliament.

Separately, Italy's government voted by 313 votes to 291 to shelve plans for new nuclear power plants. Earthquake-prone Italy currently has no reactors, but pro-nuclear prime minister Silvio Berlusconi had planned to build new nuclear plants. He decided to scrap the plans following rising public concern after the disaster at Fukushima.