Outgoing President Donald Trump, seeking to fulfill a campaign promise to end the two wars launched after the 9/11 attacks, had ordered force levels slashed in both countries to that level by January 15.

Acting Secretary of Defense Chris Miller said progress towards peace in both countries permits the cuts without a decrease in security for Americans and their counterparts.

"Today, the United States is closer than ever to ending nearly two decades of war and welcoming in an Afghan-owned, Afghan-led peace process to achieve a political settlement and a permanent and comprehensive ceasefire," he said in a statement.

More cuts, he said, depend on the progress of the peace talks between the Afghanistan government and Taliban rebels.

"Moving forward, while the (Defense) Department continues with planning capable of further reducing US troop levels to zero by May of 2021, any such future drawdowns remain conditions-based," he said.

On Iraq, Miller said the US force reduction reflects "the increased capabilities of the Iraqi security forces."

He added, though, that the reduction "does not equate to a change in US policy."

"US and Coalition forces remain in Iraq to ensure the enduring defeat of (the Islamic State group)."

"We will continue to have a counterterrorism platform in Iraq to support partner forces with airpower and intelligence," he said.