Northwest Airlines laid off 27,500 more employees Wednesday because of a strike by pilots.

The affected employees include mechanics, flight attendants, customer service employees and other ground workers. Northwest's first layoffs, announced Tuesday, covered 177 workers.Northwest also announced it has canceled all flights through Labor Day.

"This is a sad day for Northwest Airlines," said Dewayne Tucker, Northwest vice president for human resources.

Northwest said more layoffs may follow. The company has about 50,000 employees.

Earlier Wednesday, the National Mediation Board called an "exploration meeting" to bring management and union leaders back to the negotiating table to identify areas of flexibility as the pilots' strike entered its fifth day.

Both parties indicated there was room for negotiation in their Tuesday meeting with Transportation Secretary Rodney Slater, spokesman Jim Armshaw said .