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WASHINGTON, April 5 (Reuters) - The U.S. Defense Department is preparing to send another 12,000 National Guard combat troops to Iraq, NBC Nightly News reported on Thursday, citing Pentagon sources.

New orders awaiting the signature of Defense Secretary Robert Gates will put 12,000 National Guard troops on alert to prepare to deploy to Iraq, the report said.

Four Guard combat brigades from units in four states would be involved in the involuntary mobilization, NBC said.

The one-year combat deployment would begin early in 2008, the report said.

The Pentagon referred queries about the report to the National Guard, where a spokesman had no immediate comment.

Gates did not mention a possible Guard deployment at at news conference on Thursday.

More than four years into the U.S.-led war in Iraq, the U.S. military shows increasing signs of strain.

On Monday, the Pentagon said it would send another 9,000 U.S. troops to Iraq, with about half of them returning to combat ahead of schedule.

Two of the affected Army units, totaling about 4,500 troops, will return to combat short of their promised year at home, reflecting the strain placed on U.S. forces by operations in Iraq and Afghanistan.

Under the Bush administration's new Iraq policy announced earlier this year, the Pentagon has increased force levels there by about 30,000 troops in an attempt to regain control of security and reduce sectarian violence.

The units announced this week largely replace forces already in Iraq, which number around 145,000.

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