DETROIT (Reuters) - The United Auto Workers union's unit leaders on Friday unanimously approved General Motors Corp's <GM.N> proposed four-year labor contract that includes a signing bonus and a new round of voluntary buyouts for the automaker's workers.

"It was unanimously supported," a source familiar with the contract said.

The proposed contract, which has to be ratified by a majority of GM's 73,000 workers, includes a $3,000 signing bonus, a health-care trust fund to reduce the automaker's costs and a new round of buyout offers for GM workers, the source said.