Harold Hamm, Donald Trump’s top energy adviser, has urged Russia and Opec to rein in oil production in an effort to raise prices, saying US companies have already lowered output during a two-year price war that has upended the energy industry. Mr Hamm, a billionaire pioneer of the North American shale boom who has been tipped as a potential energy secretary should Mr Trump win the US presidential election, said Opec and Russia should agree to freeze production when they meet this month to discuss ways to stabilise the market.