General Motors has urged the United Auto Workers union to agree to around-the-clock bargaining to try to resolve a 25-day US strike that has cost the company over US$1bn in profits, according to a letter seen by the Reuters news agency.

The largest automaker reportedly expressed frustration after presenting the union with a comprehensive new contract offer on Monday without getting a formal counteroffer.

The UAW said in a letter earlier on Thursday that work to resolve issues by five committees must be completed first before it will make a comprehensive counterproposal, Reuters reported.

"We should engage in bargaining over all issues around the clock to get an agreement," GM vice president Scott Sandefur wrote in the Thursday letter to the UAW.

Meanwhile, the New York Post reported GM CEO Mary Barra had called a secret meeting with the UAW to end the strike.

In an unusual move union officials took as a positive step, Barra on Wednesday asked UAW president Gary Jones and Terry Dittes, the union's vice president leading the main talks, to come to a conference room near her office around 3:30 p.m., two people familiar with the meeting told The Post.

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Soon after, Jones and Dittes were escorted by armed guards to Barra's office in the Detroit Renaissance office complex, where GM is headquartered, for a meeting that lasted about half an hour, the source told the paper.

As reported earlier by The Post, GM officials on Tuesday had abruptly pulled out of a meeting for major negotiations in which officials were slated to discuss job security and bringing jobs back to the US from Mexico.

No such meeting occurred on Wednesday either, and Barra that afternoon appeared concerned that talks were off track, according to a Post source.

UAW officials had been angling for a direct meeting with Barra as a way to break through on negotiations, which have remained stalled for about five days, the paper said.

As reported by The Post, UAW top officials last week were even discussing holding a "no confidence" vote against Barra as a way to draw her into discussions. Sources told the paper union leadership believes it was Barra who personally decided to reverse a decision to remove health care coverage for striking workers.

During Wednesday's surprise meeting, it was unclear whether Barra offered or demanded any concessions, or what specific issues were discussed, the report said. Nevertheless, the meeting appeared to have injected new life into the negotiations, the paper's sources said.

Smaller groups have continued to meet to hash out details on pension benefits, pay issues and job security, two sources told the Post.