Workers at the Fukushima Daiichi nuclear power plant say they are regaining control of a reactor after its temperature rose dramatically this week, casting doubt on government claims that the facility has been stabilised.

The plant's operator, Tokyo Electric Power [Tepco] was forced to increase the amount of cooling water being injected into the No 2 reactor after its temperature soared to 73.3C earlier this week.

By Tuesday night, the temperature had dropped to 68.5C at the bottom of the reactor's containment vessel, where molten fuel is believed to have accumulated after three of Fukushima Daiichi's six reactors suffered meltdown after last year's tsunami disaster.

The temperature at the bottom of the No 2 reactor vessel had risen by more than 20C in the space of several days, although it remained below the 93C limit the US nuclear regulatory commission sets for a safe state known as cold shutdown. Tepco said it had also injected water containing boric acid to prevent a nuclear chain reaction known as re-criticality.

The operator said the sudden rise in temperature did not call in to question the government's declaration in December that all three damaged reactors had achieved cold shutdown.

"The temperature of the reactor pressure vessel seems to be close to peaking out," Junichi Matsumoto, a Tepco spokesman, told reporters.

Late last year, however, the minister in charge of the response to the Fukushima disaster, Goshi Hosono, conceded that officials had no idea about the exact location of molten uranium fuel but assumed that it had come to rest at the bottom of its containment vessels.

Hosono said the temperature rise may have been triggered by work to replace a cooling pipe after freezing weather in north-eastern Japan caused a number of water leaks at the site.

The use of bigger volumes of water to cool the No 2 reactor presents Tepco with the additional problem of a build-up of radioactive water. The utility said recently it had processed more than 220,000 cubic metres of contaminated water using treatment facilities, but added that as much as 95,000 cubic metres – enough to fill 38 Olympic-sized swimming pools – may have accumulated in the reactors' basements.

Tepco workers started injecting water into overheating reactors after the 11 March tsunami crippled the backup power supply to cooling systems.

The Fukushima Daiichi accident has led to the closure of all but three of Japan's 54 reactors, to undergo regular maintenance and new stress tests designed to gauge their ability to withstand powerful earthquakes and tsunamis.

The Yomiuri Shimbun said this week that the government planned to restart two reactors in Ohi, western Japan, before the last reactor is due to go offline at the end of April. If the restarts do not take place by then, Japan will be without a single working nuclear reactor.

But the trade minister, Yukio Edano, said that no deadline had been set to restart any reactors, a move some experts say should wait until the official investigation into the Fukushima accident has been completed.

Edano acknowledged that an early restart would be difficult, given lingering public concern over the safety of nuclear power. "The only standard is whether we can gain a certain level of understanding from the local people and the public," he said.