Nov 20 (Reuters) - Workers at MGM Resorts International's Detroit casino have rejected a tentative five-year labor contract and will continue with their month-long strike, union officials said on Monday.

Although the reason for the rejection was not clear, the Detroit Casino Council - a negotiating committee representing five unions - said the new agreement included the largest pay increase ever negotiated, including an 18% immediate raise on average.

"This is a very disappointing result, especially considering the historic nature of our offer," MGM Grand said in a letter to employees, adding that more than 600 of the council-represented workers voted to ratify the contract.

However, about 2,100 union members at Hollywood Casino at Greektown and MotorCity Casino voted on Monday to ratify the new contract, ending a 34-day strike.

The council reached a tentative agreement on Nov. 17 for a new contract covering 3,700 workers at MGM Grand Detroit, Hollywood Casino at Greektown operated by Penn Entertainment and MotorCity Casino.

The Unite Here Local 24, the United Auto Workers (UAW) and Teamsters Local 1038 were among the unions that negotiated a contract with the companies.

Despite the continuation of the strike at MGM Grand, operations will continue as usual, the casino said.

About 40,000 hospitality workers in the Las Vegas Strip also reached a tentative labor deal with casinos operated by Wynn Resorts, Caesars Entertainment and MGM earlier this month. (Reporting by Ananta Agarwal; Editing by Anil D'Silva)