DETROIT -- In an effort to conserve cash, General Motors will require thousands of salaried workers to take two additional vacation days in December. Those days will be taken on top of GM's two-week holiday shutdown next month.

The moves come after GM said on Friday it is battling "unprecedented economic and credit market turmoil'' and warned that it may not have enough cash to operate by mid-2009.

GM ended the third quarter with $16.2 billion in cash on hand, down from $21 billion on June 30. The net quarterly loss was $2.5 billion, including one-time gains and charges. Before those special items, GM reported an adjusted loss of $4.2 billion. Revenue fell $5.8 billion, to $37.9 billion, and global unit deliveries dropped 11 percent.

GM now is looking for another $5 billion in savings by the end of next year, on top of $15 billion earmarked in July.

In a letter sent Nov. 5, all employees at GM's Vehicle Engineering Center in suburban Detroit, the proving ground in Milford, Mich., and the desert proving ground near Mesa, Ariz., were told they would be required to take Dec. 22 and Dec. 23 off, says GM spokesman Dan Flores.

Before the announcement, employees at those sites were scheduled to work those two days, and the holiday started Dec. 24, Flores says.

"We notified the employees last week that in our continued efforts to conserve cash we are asking employees to take two additional vacation days," Flores says. "It'll give us significant savings in utilities and facility expenses."

Employees who have taken all of their vacation time this year will be required to take the time off and will have two fewer vacation days next year, Flores says.

GM employs between 16,000 to 17,000 workers at its engineering center, Flores says. He did not have the employee count at the two proving grounds.

He says world headquarters and manufacturing plants will remain staffed through December.

GM does not plan to shut any of its factories for an additional two days in December.