By Tim Hepher and Alana Wise

PARIS/NEW YORK (Reuters) - United Airlines <UAL.N> is in talks with planemakers Airbus <AIR.PA> and Boeing Co <BA.N> over the purchase of wide-body, long-haul passenger jets to replace a fleet of 50 Boeing 767 aircraft, people familiar with the discussions said on Tuesday.

Europe's Airbus is expected to offer its upgraded A330neo jet against Boeing's 787 Dreamliner in a deal potentially worth some $14 billion at list prices, the two sources said. All three companies declined to comment on the nature of the talks.

A win in the jet production contest would be especially important for Airbus, whose struggling A330neo program has lost a number of recent bids to U.S. rival Boeing for wide-body, long-haul aircraft.

Hawaiian Airlines in March chose Boeing to supply 10 new 787-9 aircraft, dropping an earlier order of the A330neo.

The European planemaker was again bested last month when American Airlines, the world's largest carrier, chose Boeing to supply 47 new 787 Dreamliner jets.

The fate of an order for 28 A330neos from IranAir is also in jeopardy, on concerns over a Tuesday decision by U.S. President Donald Trump to withdraw the United States from an international nuclear deal.

Airbus Americas Chairman Jeff Knittel said Airbus remained confident in the A330neo despite recent setbacks.

"This is a marathon, not a sprint," he told Reuters.

"We are talking to a number of people as potential customers. We think the airplane makes sense and we are optimistic."

He declined to say whether United was among the airlines with which Airbus is involved in discussions.

The Chicago-based carrier's fleet already includes Boeing 787s, and the carrier has firm commitments for an additional 14 jets scheduled for delivery through 2027.

It does not fly the A330 family, the latest version of which - the A330neo - is due to enter service this summer.

(Reporting by Tim Hepher, Alana Wise; editing by Hugh Lawson and Lisa Shumaker)