(Adds troop increase report, paragraphs 5-7)

WASHINGTON, Aug 3 (Reuters) - U.S. Defense Secretary Robert Gates held an unannounced meeting with his top commanders in charge of the war in Afghanistan at a U.S. air base in Belgium, the Pentagon said on Monday.

The meeting on Sunday at Chievres Air Base was arranged to give Gates an update on a review of the war being conducted by U.S. Army General Stanley McChrystal, the top U.S. and NATO commander in Afghanistan, Pentagon press secretary Geoff Morrell said.

"He wanted an opportunity to speak firsthand with his commanders and his senior military advisers about how the assessment was unfolding," Morrell told reporters.

The unusual meeting was not announced in advance. In addition to McChrystal and Gates, the meeting was also attended by a handful of other top U.S. military and defense officials, Morrell said. He declined to discuss further details.

The officials discussed a possible troop increase in Afghanistan, ABC News reported, citing defense sources who it did not identify.

McChrystal's advisors have said publicly that he will need more boots on the ground even after force levels reach 68,000 U.S. troops -- on top of more than 30,000 from allied nations -- later this year.

The ABC report said McChrystal's assessment team has suggested deploying about 12,000 to 27,000 additional U.S. troops, above the more than 20,000 extra troops President Barack Obama already has approved.

The Obama administration has poured tens of thousands of troops into Afghanistan this year in an effort to turn the tide in a war that U.S. officials have acknowledged they are not winning. The war began in 2001.

McChrystal took command in mid-June with orders to revitalize the war effort after Gates dismissed McChrystal's predecessor, U.S. Army General David McKiernan. McChrystal is due to provide his 60-day assessment of the war by mid-August.

Morrell said the meeting also was attended by: Admiral Mike Mullen, chairman of the Joint Chiefs of Staff; U.S. Army General David Petraeus, head of U.S. Central Command; Admiral James Stavridis, NATO's top commander; Pentagon policy chief Michele Flournoy; and U.S. Army Lieutenant General David Rodriguez, McChrystal's deputy. (Reporting by Andrew Gray and JoAnne Allen; Editing by Will Dunham)

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