WASHINGTON - President Barack Obama on Monday added to his schedule an evening session with top advisers on Afghanistan as he closes in on a decision on whether to send thousands more U.S. troops.

The White House said Obama would meet with Vice President Joe Biden, Secretary of State Hillary Clinton, Defense Secretary Robert Gates and other officials at an 8 p.m. EST/0100 GMT Tuesday meeting in the Situation Room.

It will be the ninth such meeting as Obama nears a decision on whether to add as many as 40,000 troops to an eight-year-old war that was started after the September 11 attacks and that has begun to try the patience of the American people.

With the U.S. capital shutting down for the Thanksgiving holiday this week, Obama is not expected to announce his plans until next week at the earliest.

In a sign that he is in the late stages of his deliberations, NATO officials said the alliance will hold a meeting on December 7 to raise forces for Afghanistan, with the expectation that Obama will have laid out his plans for U.S. troop levels by then.

There are about 110,000 foreign troops, including 68,000 U.S. soldiers, in Afghanistan fighting a stepped-up Taliban insurgency.

The president has been reviewing war strategy in Afghanistan for the past two months after Army General Stanley McChrystal, the top U.S. commander there, said in a report to him that conditions were deteriorating and more troops were needed.

AMERICANS DIVIDED

Obama's top national security advisers, including Gates and Admiral Mike Mullen, chairman of the Joint Chiefs of Staff, are believed to have rallied around options that would send 30,000 to 40,000 more troops and trainers.

The total could approach the 40,000 requested by McChrystal.

Obama faces conflicting pressures on Afghanistan. Americans are divided about whether to send more troops. Republicans in the U.S. Congress insist more troops are needed to prevent a Taliban resurgence, while his own Democrats in general would like to see the United States find a way out of Afghanistan.

Two veteran Democratic lawmakers have called for imposing a war tax to pay for a troop surge. The two were David Obey, chairman of the House of Representatives Appropriations Committee, and Carl Levin, chairman of the Senate Armed Services Committee.

But Democratic Senator Daniel Inouye, chairman of the Senate Appropriations Committee, opposed it. Someone has to demonstrate how it can be done, Inouye said.

A Washington Post-ABC News poll found last week that 46 percent of Americans support a large influx of troops to fight insurgents and train the Afghan military, while 45 percent back a smaller number of new U.S. forces more narrowly focused on training.

Obama and his advisers have debated options ranging from sending tens of thousands more troops to limiting troop increases and concentrating on attacking al Qaeda targets.

One factor that has complicated the deliberations has been concerns about corruption in Afghan President Hamid Karzai's government. Obama has said he wants to ensure he has a reliable partner there.

(Additional reporting by Andy Sullivan and Adam Entous, editing by Eric Beech)