* Questions still remain on when announcement will come
* Obama's decision not influenced by "outside factors" (Adds details, background)
WASHINGTON, Oct 31 (Reuters) - U.S. President Barack Obama is unlikely to make a decision on his Afghanistan strategy and sending thousands more troops there before he embarks on his trip to Asia on Nov. 11, a senior administration official said on Saturday.
The official, who spoke on condition of anonymity, stressed that while a decision was unlikely before then, it had not been ruled out.
With violence this year reaching its worst levels in Afghanistan since the Taliban was ousted by U.S.-backed Afghan forces in 2001, Obama is under pressure to spell out U.S. plans.
It is unclear if Obama would wait until he returned from his Nov. 11 -20 trip to unveil his decision or announce it while he was traveling in the region.
The timing of Obama's announcement would not be influenced by outside factors, the official said, referring to the political upheaval in Afghanistan after a disputed presidential election in August. A run-off is scheduled for Nov. 7.
Western diplomatic sources said President Hamid Karzai's election rival, Abdullah Abdullah, was leaning toward pulling out of the run-off. Abdullah will announce his decision on Sunday.
Obama has faced criticism from Republican opponents, including former Vice President Dick Cheney, over his lengthy review of the strategy he put in place in March. Critics say he is being over-cautious and the delay in making an announcement on the way forward in Afghanistan is emboldening the Taliban.
The White House says Obama will not be stampeded into making a hasty decision. It accuses the former Bush administration of neglecting the eight-year-old war and allowing the security situation there to deteriorate.
Obama held talks with the Joint Chiefs of Staff, the leaders of each branch of the U.S. military, at the White House on Friday. He is expected to hold a further meeting with his security team next week. So far, he has held eight meetings.
The Pentagon is looking at the resources it would need to fulfill the strategic options being considered, the administration official said. But Obama had not asked the Joint Chiefs to come back to him with specific troop numbers.
General Stanley McChrystal, the commander of U.S. and NATO forces in Afghanistan, has recommended deploying an additional 40,000 troops next year, a figure that includes trainers to accelerate the expansion of the Afghan army.
Another and potentially more politically palatable option under consideration by the administration would add about 10,000 to 15,000 troops, a large portion of whom would be focused on increasing the training of Afghan forces, a top priority for Obama's Democratic allies in Congress.
Officials say Obama may opt for a number in between.
There are about 67,000 U.S. troops and 42,000 allied forces in Afghanistan. (Reporting by Ross Colvin; Editing by Peter Cooney)
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