An unexpected bidder emerged Tuesday for the German lignite mining operations of Swedish utility Vattenfall: Greenpeace.

State-owned Vattenfall has been pondering strategies for its lignite operations in eastern Germany, whose sustainability was questioned in a German Institute for Economic Research (DIW) study last year.

In a statement, Greenpeace said it had put in the bid to try to prevent the opening of new mines "and thus keep the lignite in the ground".

The soft brown coal has a dirty reputation because of its high emissions of heat-trapping carbon dioxide (CO2).

"If the operation were sold to another buyer it would lead to the opening of five new lignite mines," Greenpeace said.

"These mines contain an amount of carbon dioxide that would be equivalent to 1.2 billion tons."

Vattenfall said Greenpeace had filed a letter of interest with Citigroup, which has been mandated to oversee the sale, and it would take a close look at the bid.

"This is an open process and all serious offers are welcome," said Vattenfall spokeswoman Sabine Froning.

The company did not give financial details of the offer.

Vattenfall, the third-largest electricity producer in Germany, last month launched moves to move out of extraction and production of lignite in Germany and decommission power plants and coal mines to help meet carbon reduction targets.

In April, Greenpeace said it hoped to see a phaseout from lignite through "a climate-friendly and socially responsible road map."

"We are seriously going to discuss the purchase with Vattenfall. We have good knowledge of the issue of the future energy market and the evolution of climate policies," Greenpeace Sweden president Annika Jacobson said.

The east German operations employ around 8,000, in a region which still far lags western Germany in development. The lignite industry provides some 85,000 jobs in Germany as a whole.

The move comes ahead of December's UN-led climate talks in Paris, targeting a global accord to limit global warming to 2 degrees Celsius (3.6 degrees Fahrenheit) over pre-Industrial Revolution levels.

Those talks come amid heady debate the future of fossil fuels, with some analysts saying many coal mines could become stranded assets, which cannot be exploited because of environmental costs.

Greenpeace said the Swedish state, which has 100-percent ownership of Vattenfall, bore heavy responsibility for protecting the environment.

"The Swedish government cannot contribute to an acquisition by a buyer that would continue to burn enormous amounts of coal. This is a signal Sweden cannot give before the Paris climate conference," it said.

Greenpeace estimated its reserves at 18 million euros ($21 million) in its last annual accounts.

Two Czech firms have already expressed early interest for the Vattenfall operations.