Kazakhstan has imposed a fine of more than half a billion dollars on an oil consortium led by Chevron, in a move that threatens to worsen the investment climate in the resources-rich Central Asian republic.

The fine for ecological violation at the Tengiz oilfield comes after the Kazakh government imposed a fine and stopped work at the Eni-led consortium developing the large Kashagan oilfield.

Chevron’s fine comes ahead of a key visit this weekend by Romano Prodi, the Italian prime minister, to Astana to help resolve the row between an Eni-led consortium and the Kazakh government.

Nurlan Iskakov, the Kazakh ecology minister, said the government imposed a $609m fine on the Chevron-led Tengizchevroil venture. “Any company that does not fulfil ecological requirements will be dealt with in the harshest way,” Mr Iskakov said.