Large US-listed shale player Magnum Hunter Resources has launched a rival takeover offer for Cooper Basin junior Ambassador Oil and Gas , outbidding Drillsearch Energy by 20 per cent.

Magnum Hunter, with a market ­capitalisation of about $US1.75 billion ($1.87 billion), will offer one share for every 27.8 shares in Ambassador, valuing the target at 34¢ per share.