By Alberto Sisto and Deepa Babington

ROME (Reuters) - A major Alitalia (AZPIa.MI: Quote, Profile, Research) union abandoned talks with Air France-KLM on Monday, in the latest blow to the French carrier's efforts to win labor support for its planned takeover of the nearly bankrupt Italian airline.

Barely two weeks since striking the deal, Air France-KLM's (AIRF.PA: Quote, Profile, Research) agreement to buy Alitalia risks falling apart due to stiff opposition from unions, from Milan's airport operator and from the center-right party expected to win April elections.

Time is running out for the carriers. Alitalia extended the deadline for the talks' conclusion to Wednesday -- two days beyond an initial March 31 cut-off.

The UIL, one of Alitalia's biggest unions, walked out of Monday's talks after demanding a suspension until after the election on April 13-14, a union official said.

"It's a false and useless negotiation," UIL General Secretary Luigi Angeletti told Italian television, noting that Italy's next government would have to approve the deal and Milan's airport operator SEA would have to withdraw a $2 billion lawsuit against Alitalia before the takeover could be concluded.

The meeting was briefly suspended but soon resumed without the UIL representatives, the official said. The carriers convened a new meeting with the unions for Tuesday morning, but it was not known whether UIL would attend.

Other union officials at the meeting voiced different opinions on the progress in talks. Some spoke of "timid openings" while others said that talks had stalled and that Air France-KLM CEO Jean-Cyril Spinetta "keeps repeating the same things."

Union support for the deal is a prerequisite to Alitalia obtaining a 300 million euro bridge loan from the Italian government to continue operations until the takeover is wrapped up, a union official quoted Alitalia's chairman as saying.