Rupert Murdoch's 21st Century Fox Inc has said it offered to buy Time Warner Inc, a move that would unite two of the world's most powerful media conglomerates, but Time Warner rejected its offer.

Time Warner's stock jumped 17.3% to $83.33 on the New York Stock Exchange after news that Mr Murdoch had his sights on Time Warner.

The company owns the Warner Bros movie studio and cable channels such as HBO and CNN, among other media properties.

Mr Murdoch's cash-and-stock bid was worth about $80bn, or $85 per share, people familiar with the matter told Reuters.

The offer, first reported by The New York Times, consisted of 60% in stock and the rest in cash.  

21st Century Fox later confirmed it had made a formal takeover proposal in June but said there were no talks currently under way.

Even so, Mr Murdoch and his advisers are unlikely to abandon his ambition to put Time Warner in his empire so easily, one of the people said, pointing out that he has the "disciplined determination" to get a deal done.

Fox's overtures to Time Warner could accelerate a wave of consolidation that is already reshaping the US media landscape.

Reuters reported this month that Mr Murdoch was in the midst of a deal that would give Fox the firepower to buy a content company.

Fox, which owns cable news channel Fox News as well as movie studio 20th Century Fox, has indicated it would sell CNN as part of its proposal to buy Time Warner to clear any regulatory hurdles, according to the people familiar with the matter.

Fox currently estimates that a combined company would save $1bn in costs and possibly more, primarily by cutting sales staff and back-office functions, the people familiar with the matter said.

It believes that detailed negotiations with Time Warner could reveal much higher synergies than $1bn, which may justify Fox improving its offer, the people said.

The combined company's revenue would be more than $60bn.