Facing a deadline and an ultimatum from San Francisco's mayor, the operators of 14 hotels at the center of a protracted labor dispute refused Tuesday to end their four-week lockout of unionized employees while contract negotiations proceed.

"We respectfully decline the mayor's request for a 90-day cooling off period," Cornell Fowler, a spokesman for the hotel operators, said after they delivered their message to Mayor Gavin Newsom.

Newsom on Monday threatened to call for a boycott of the hotels if they didn't agree to his request for the cooling off period that would have allowed locked out workers to return to their jobs while talks on the contract affecting room cleaners, bellmen, cooks and others continue.

Expressing frustration that the dispute was tarnishing the city's image and hurting its workers, Newsom said he would join hotel employees workers on their picket lines and encourage mayors in other cities to play hardball with the properties' corporate owners.

"This is our No. 1 private industry in our city, and I have great respect for these hoteliers," Newsom told The Associated Press in a phone interview. "But I'm not going to allow other people to pull the strings and damage the city because they are putting their interest over the interest of the city."

The hotel workers union, Local 2 of Unite Here, called a strike at four of the hotels on Sept. 29, and the hotels responded by locking at workers at the 10 others two days later. The workers ended the strike last Wednesday, but the 14 hotels declared the lockout would continue until agreement on a new contract was reached.

Fowler said the hotel operators were aware of the Newsom's warnings, but concluded they did not want to run the risk of another strike. They have continued to operate, although at some places with reduced room service and restaurant hours, with replacement workers.

"There is no guarantee after 90 days they wouldn't strike and it would be too expensive to ramp up again," Fowler said.

The workers' five-year contract expired in August. Major sticking points are whether the next agreement should last five years or two years, wages and health care costs.