Gabriel, environment minister since November, told a news conference that existing reserves of uranium could run out in 30 to 40 years.

"The technology is expensive and the fuel relatively cheap, but the latter will change fairly soon," Gabriel said.

He added that those advocating a longer life for the country's nuclear power stations were simply seeking to increase earnings for the affected firms. E.ON, RWE, EnBW and Vattenfall Europe operate nuclear plants in Germany.

Gabriel's stance appears to have the backing of Chancellor Angela Merkel, who must hold the fragile coalition of left and right together. Government spokesman Thomas Steg said on Wednesday Merkel had indicated she would stick to a coalition deal which foresees an exit from nuclear power.

The Bavarian Christian Social Union (CSU) and its Christian Democrat (CDU) allies in neighbouring Baden-Wuerttemberg, home to a number of nuclear plants, want the nuclear debate reopened following a gas dispute between Russia and Ukraine.

RELIANCE ON RUSSIA

Roughly a third of German gas comes from Russia, and the dispute over pricing, which led Moscow to cut off the gas flow through a key pipeline for two days, has many asking whether Germany is too reliant on Russia.

"I am confident that we will be able to enter a discussion (on nuclear power) soon," CSU chief Edmund Stoiber told a party meeting on Thursday. "But we cannot force it."

Stoiber said he was not trying to reverse Germany's abandonment of nuclear power, but simply to prolong the life of safe atomic plants.

Gabriel said 25 percent of Germany's electricity could come from renewables by 2020. By then, energy productivity should have doubled compared with 1990s levels, he added.

Nuclear power, which became unpopular in Europe after the 1986 Chernobyl disaster, has been making a comeback. The first new nuclear plant on the continent in years is being built in Finland. One reason for nuclear's return to favour is the fact that nuclear reactors emit virtually no greenhouse gases.