WASHINGTON, April 14 (Reuters) - U.S. ability to collect intelligence and act against extremist threats that arise in Afghanistan will diminish after the departure of U.S. troops, CIA Director William Burns warned on Wednesday.

Burns’ testimony to the Senate Intelligence Committee highlighted a key risk inherent in President Joe Biden’s decision to pull all remaining U.S. forces out despite the enduring presence of al Qaeda and Islamic State militants.

The former top diplomat, who took over as CIA chief last month, spoke hours before Biden announced that all remaining U.S. troops would leave Afghanistan before Sept. 11, the 20th anniversary of the al Qaeda attacks that triggered the U.S.-led invasion.

“When the time comes for the U.S. military to withdraw, the U.S. government’s ability to collect and act on threats will diminish. That’s simply a fact,” Burns said.

Burns added, however, that the U.S. government would retain capabilities after the withdrawal to “help us to anticipate and contest any rebuilding effort” by al Qaeda.

A United Nations report in January said there were as many as 500 al Qaeda fighters in Afghanistan and that the Taliban maintained a close relationship with the Islamist extremist group. The Taliban deny there are any al Qaeda operatives in Afghanistan.

Biden said the United States would monitor the terrorist threat, reorganize counterterrorism capabilities and keep substantial assets in the region to respond to threats to the United States emerging from Afghanistan after the troop departure.

“There’s probably not a space on the globe that the United States and its partners can’t reach,” U.S. Defense Secretary Lloyd Austin told a news conference at NATO headquarters in Brussels.

But Burns’ warning reflects many experts’ concerns that the withdrawal will leave U.S. intelligence officers with significantly less security, constraining their ability to collect timely information on the ground.

“The U.S. will lose, to a great extent, our ‘eyes-on’ capabilities to deal with the threat. The U.S. also will no longer be able to cooperate directly with the Afghan security forces,” said Lisa Curtis, who served as the top White House adviser on Afghanistan in the Trump administration.

She questioned where U.S. counter-terrorism forces would be repositioned outside of Afghanistan, citing a “history of difficulties” basing U.S. forces in the “most suitable locations” of Pakistan and Uzbekistan.

Spy agencies rely on U.S. troops for critical support like security and medical evacuation, said Marc Polymeropoulos, a former top CIA operations officer who served in Afghanistan.

“The idea that the intelligence community can stay with the U.S. military leaving? I just don’t see how that’s possible,” he said. “You’re almost going to a pre-9/11 collection posture in Afghanistan, which didn’t turn out so well.”

The problem of dealing with new threats, he warned, will worsen if - as some U.S. officials and experts fear - a full-scale civil war erupts after the U.S. withdrawal because Afghan intelligence and special forces will be “fighting for their own survival.”