General Motors Corp and the United Auto Workers union broke off negotiations early on Friday and agreed to return to the bargaining table later in the day, as 73,000 GM workers returned for another day of work without a new contract.

Talks adjourned around 12:45 a.m. EDT on Friday, GM spokesman Tom Wickham said, as the two sides adjourned a bargaining session of more than 14 hours.

The largest U.S. automaker and its major union have been locked in intensive labor negotiations since last Friday, when the UAW's past contract expired.

The talks have failed to reach a breakthrough on the thorny issue of healthcare costs and could continue into next week, people briefed on the progress of the talks told Reuters Thursday night.

The outcome of the contract talks is seen as crucial to efforts by the three Detroit-based automakers -- GM, Ford Motor Co and Chrysler LLC -- to recover from combined losses of $15 billion last year and sales difficulties that have driven their share of the U.S. market below 50 percent.

Progress is very slow, Al Benchich, president of Local 909 in Warren, Michigan, said in a message posted on the local unit's Web site, which was not updated since Wednesday night.

Since talks started two months ago, negotiations have hinged on how fully GM should be required to fund a special trust -- known as a voluntary employee beneficiary association, or VEBA -- in exchange for clearing an unfunded liability estimated at about $50 billion from its balance sheet.

One local union official told members that financial experts had been brought in on Tuesday.

Outside financial consultants were brought in to look at all the numbers and funds, Enrique Flores Jr., president of UAW Local 276 in Grand Prairie, Texas, said in a message on the unit's Web site. The financial consultants will be going over the finances for a while.

The UAW has retained investment bank Lazard as an adviser, bringing back the firm that helped the union negotiate mid-contract concessions with GM in 2005.

For GM, key issues include cutting health-care costs and establishing a two-tier wage system that would allow the automaker to cut wage and pension costs as its aging work force retires, people familiar with the talks have said.

To offset those concessions, the UAW has sought job security guarantees and a substantial signing bonus for the GM workers it represents, according to these people, who asked not to be named because they were not authorized to discuss the private talks.

Representatives from the UAW and GM have declined to comment on the content of the private talks.

UAW President Ron Gettelfinger and Vice President Cal Rapson, who are leading the union negotiations with GM, cautioned earlier this week the union would set a firm deadline for talks to conclude if progress stalled.