At the stricken Fukushima Daiichi plant in the northeast of the country, engineers were struggling to find a new way to cool one of the six crippled reactors and Japan's Nuclear and Industrial Safety Agency said it was now "highly likely" there was a hole in the suppression unit of the reactor.

Mr. Kan, whose public support stands at about 30 percent, had sought a grand coalition to help the country recover from its worst ever natural disaster and enact bills to pay for the country's biggest reconstruction project since World War Two.

His Democratic Party controls parliament's lower house but needs opposition help to pass bills because it lacks a majority in the upper chamber, which can block legislation.

But the head of the main opposition Liberal Democratic Party (LDP), who last week ruled out joining hands, pressured Kan to go.

"The time has come for (the prime minister) to decide whether he stays or goes," Kyodo news agency quoted Sadakazu Tanigaki as telling a news conference.

Tanigaki's comment reflects the view of many in his conservative party that Kan must step down as a precondition for any coalition as well as a hope that criticism of Kan within his own Democratic Party will gather steam after party powerbroker Ichiro Ozawa blasted the premier over his crisis management.

Upper House speaker Takeo Nishioka, a well-known Kan critic from the Democrats, also urged Kan to resign, Kyodo said.