General Motors' use of temporary workers is a key bargaining issue in negotiations between the automaker and the striking United Auto Workers union over a new four year contract, a media report said.

Reuters reported details of proposals had not been disclosed, noting the union approved the occasional use of temporary workers by US automakers during the last contract talks in 2015 when the two sides agreed to gradually phase out the existing two-tier wage structure.

The news agency noted temporary workers account for about 20% of the workforce at Japanese automakers in the US and are paid an average US$50 an hour, citing data compiled by Automotive News and the Center for Automotive Research.

In comparison, GM pays its workers an average $63 an hour, Reuters added, while temps, who typically earn less than $20 an hour and have limited benefits, account for about 7% of its U.S. workforce.

GM has said it can reduce labour costs and safeguard union jobs by using more temporary workers.

Reuters noted the use of temporary workers also allows the Detroit automakers to cover for high rates of absenteeism among UAW workers. The companies want the UAW to agree to stricter rules to deter workers from not turning up for their shifts.

The union, which has seen its membership dwindle to less than 400,000 from a peak of 1.5m in 1979, granted US automakers numerous concessions 10 years ago to help the industry weather the global credit crisis and GM and Chrysler bankruptcies.

Reuters also noted one of the biggest 'givebacks' was an agreement in 2009 to end the controversial jobs bank, which guaranteed laid off workers would receive up to 90% of their regular pay. The jobs bank was approved in 1984 and expanded in 1990. Ultimately, it cost US automakers billions of dollars to keep unemployed workers on their payrolls.

The two sides also are wrangling over how much union members should pay for spiraling health care costs, which once were covered entirely by the manufacturers, the news agency's report added.