Germany has been forced to restart mothballed coal plants after Russia curtailed its energy supplies to the country in the wake of its invasion of Ukraine.

Its decision to extend the lifetime of two but not a third nuclear plant beyond their planned shutdown at year's end has however led to a split within Chancellor Olaf Scholz's coalition.

Economy Minister Robert Habeck of the Greens has come under pressure over his nuclear policy, with some ecologists criticising him for failing to keep to the planned atomic phase-out.

At the same time, Finance Minister Christian Lindner of the liberal Free Democrats is leading the charge in pressing for the third nuclear plant to stay on the grid beyond the end of the year.

Asked about Habeck's decision in an interview with ARD broadcaster, Thunberg said that "if we have (the nuclear plants) already running, I feel it's a mistake to close them down" if coal was the alternative.

Lindner immediately took to Twitter to welcome Thunberg's position.

"In this energy war, everything that creates electricity capacity must be kept on the grid," he said.

Nuclear power is a hot button topic in Germany's political landscape.

Former chancellor Angela Merkel had pushed through Germany's nuclear exit in the wake of Japan's Fukushima disaster.

The ecologist Greens had lent strong support then to the move, as they have their roots in Germany's anti-nuclear movement.