Washington's ability to collect intelligence and act on threats will diminish when US troops leave Afghanistan, CIA Director William Burns said on Wednesday ahead of an expected announcement of a pullout by President Joe Biden.

Burns' testimony to the Senate Intelligence Committee underscored a key risk inherent in Biden's decision to pull remaining US forces out, given the enduring presence of al Qaeda and Islamic State militants in the country.

"When the time comes for the US military to withdraw, the US government's ability to collect and act on threats will diminish. That's simply a fact," he told the committee, adding that the United States would however retain "a suite of capabilities".

US officials on Tuesday said that Biden on Wednesday would announce that all remaining US troops would leave Afghanistan before Sept 11, the 20th anniversary of the al Qaeda attacks on the United States that triggered the US-led invasion.

The Biden administration will keep "intelligence and military capabilities" in the region to deal with any emerging threats, though al Qaeda "does not currently possess" the capability for attacks on the US homeland, a senior adminstration official said.

But Burns' warning reflected some experts' concerns that the departure of US forces will leave US intelligence officers with significantly less security, constraining their ability to to collect timely information on the ground.