KGHM, one of Europe`s biggest copper and silver producers, became the owner of the US mines in 2012, when it bought rival Canadian Quadra FNX.

Copper, a highly efficient conductor of electricity and heat, is in high demand for use in renewable energy systems to generate power and in electric vehicles. New deposits are rare and increasingly difficult to recover.

The two US copper mines, in Nevada and Arizona, are valued at around $500-million and with strong copper prices they are expected to attract interest from North American private equity firms or smaller mining companies, one of the sources said.

"Scenarios for individual assets are at various stages of advancement," KGHM said in an emailed statement, adding it would continue to work on enhancing the value of its key assets.

KGHM said the $500-million valuation given by the source "appears to be at odds with the company estimation", declining to comment further.

The company has been mining in Poland since the 1950s and it is 32% owned by the government.

KGHM`s international expansion was questioned by the conservative Law and Justice (PiS) party, which took power in 2015.

Since then, KGHM has put some of its foreign assets under review for potential sale and stressed it would focus mostly on investing in core and domestic assets, including copper and silver mines and copper smelters.

Investors have also been keen for details on KGHM`s plans regarding Chilean Sierra Gorda mine, its flagship foreign asset, where it has a 55% stake, after its Japanese partner Sumitomo Metal Mining put its 45% on the block last October with the help of RBC Capital Markets.

Sources at the time said KGHM would not take on the rest of the mine.

In February, CEO Marcin Chludzinski told Reuters that KGHM sees its stake in Sierra Gorda as "optimal".