"If the conditions permit it look we would love to get people out earlier and I think that's the desire the president was expressing".
USA officials, including Milley and Secretary of State Mike Pompeo, have vowed that withdrawal would be "conditions-based", including a reduction in violence - although that commitment isn't explicit in the agreement.
Speaking via Zoom at the Aspen Institute Friday morning, O'Brien said the president has set a timeline for troop withdrawal and in the early part of next year the USA will be down to 2,500. "I'm going to engage in the rigorous analysis of the situation based on the conditions and the plans that I am aware of, and my conversations with the president", Milley said.
Milley dismissed the 2,500 troops plan revealed by O'Brien last week as "speculation" in a Sunday interview with NPR.
Defense officials insist there are no plans to have all troops home from Afghanistan by the holidays or the end of the year.
Trump's original tweet about Christmas alarmed Pentagon and State Department officials who fear that putting a definitive date on troop withdrawal could undercut negotiations to finalize ongoing peace negotiations between the Taliban and representatives of Afghan society, including the current Afghan government.
Bradley Bowman, senior director of the Center on Military and Political Power at the Foundation for Defense of Democracies, told VOA the "last thing" the US would want would be for USA equipment or bases in Afghanistan to fall into the hands of the Taliban.
Milley said anyone 'can speculate as they see fit. Commander of U.S. Central Command Kenneth McKenzie said last month that U.S. troop presence in Afghanistan would be decreased to about 4,500 by early November. I'm not going to engage in speculation.
"That was the decision of the President on a conditions-based withdrawal", Milley added, saying, "The key here is that we're trying to end a war responsibly, deliberately, and to do it on terms that guarantee the safety of the U.S. vital national security interests that are at stake in Afghanistan".
O'Brien grew testy when asked about Milley's comment.
O'Brien stated that It is not his practice to speculate, 'Other people can interpret what I say as speculation, but... I wasn't speculating today.
He said the number of troops would be reduced to less than 5,000 in the next month or so, and in the early part of next year, it will drop to 2,500. "I don't think they like fighting ISIS-K any more than we do, so I think there's some opportunities there".
"I can guarantee you".
Multiple U.S. officials said Friday that the Pentagon has not received an order to cut troops to 2,500.
That deal began a phased withdrawal of American forces and the closure of USA bases in exchange for Taliban commitments to begin peace talks with the Afghan government and deny safe haven to al Qaeda or other terrorists.
"We've had indications from our adversaries in the Taliban that they'd like to see [counterterrorism] cooperation in Afghanistan", he said.
America's exit from Afghanistan after 19 years was laid out in a February agreement Washington reached with the Taliban.
At its peak in 2011, the USA had about 100,000 troops in Afghanistan. Trump campaigned on "ending endless wars", O'Brien added, saying, "We can't spend the best of American youth in Afghanistan trying to work out a situation that's been going on for thousands of years, these tribal wars".
US forces are now drawing down from 8,600 troops to 4,500, almost eight months after the Trump administration signed an agreement with the Taliban, the militant group that once controlled Afghanistan and sheltered the al Qaeda operatives responsible for the September 11 attacks.