Carlyle Group and Riverstone Holdings, two US private equity groups, have sweetened their offer for Petroplus after earlier bids failed to win over shareholders of the Dutch energy refinery.

RIVR, the bidding vehicle representing Carlyle, Riverstone and members of Petroplus' management, raised their previous take-private offer by €1 to €9 per share, valuing Petroplus' equity at €278m ($364m), an increase of 13% on the previous offer in May.