WASHINGTON (AP) — President Donald Trump's national security adviser on Friday doubled down on his assertion that the number of U.S. troops in Afghanistan will shrink to 2,500 early next year, while suggesting that Trump's tweet that all forces should be home by Christmas was more a wish than a reality.

Seeking to clarify a series of confusing statements about the American footprint in Afghanistan, Robert O'Brien appeared to take a shot at Gen. Mark Milley, chairman of the Joint Chiefs of Staff. Milley in recent days said that the U.S. is executing a plan to reduce the number of troops to 4,500 in November, but talk of any further reduction would be “speculation.”

Confusion about troop withdrawals from America's longest war — an emotional topic for the troops and their families — began Oct. 7, when Trump tweeted that “we should have the small remaining number of our BRAVE Men and Women serving in Afghanistan home by Christmas.” When asked about those comments, O’Brien said Friday that Trump was just expressing a hope.

“I think what the president was doing is, he was expressing the same desire I think every president since the Revolutionary War has said," O'Brien said during an online event hosted by the Aspen Institute. "Whenever we’re at war, whether it was the Revolutionary War, the Civil War, World War I or World War II, all presidents ... want the troops home by Christmas.”

Instead, O'Brien said, “We’re on a path right now that looks like about 4,500 this fall and a smaller number in January and February, but if the conditions permitted, look we’d love to get people out earlier." And, echoing comments he made last week, O'Brien said, "in the early part of next year, we’re going to be down to 2,500 troops.”

Defense officials insist there are no plans to have all troops home from Afghanistan by the holidays or the end of the year. U.S. officials also said Friday that there currently is no approved plan to reduce the number to 2,500 by early next year. The officials spoke on condition of anonymity to discuss internal deliberations.

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. They also worry that a hasty withdrawal could force the U.S. to leave behind sensitive military equipment. And they continue to stress that the Taliban have still not met requirements to reduce violence against the Afghans, a key element of the U.S. withdrawal plan.

When asked about the tweet and plans to go to 2,500 by early next year, Milley told National Public Radio several days ago that the Pentagon had a conditions-based plan for troop withdrawal. He said he would not speculate beyond saying that there are 5,000 to 6,000 U.S. troops in the country now and and that number is to be reduced to 4,500 in November.

“I think that Robert O’Brien or anyone else can speculate as they see fit," Milley said. "I’m not going to engage in speculation. 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.”

O'Brien grew testy when asked about Milley's comment.

“It’s not my practice to speculate,” O'Brien said. “So other people can interpret what I say as speculation or not, but I wasn’t speculating then. I wasn’t speculating today. When I’m speaking, I’m speaking for the president.”

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 can guarantee you. ... It’s not speculation. That’s the order of the commander in chief.”

Multiple U.S. officials said Friday that the Pentagon has not received an order to cut troops to 2,500. U.S. military leaders have consistently said that any reduction below 4,500 needs to be based on conditions on the ground, including a measurable reduction in attacks by the Taliban on Afghan troops. And they say they have not seen that yet.

America’s exit from Afghanistan after 19 years was laid out in a February agreement Washington reached with the Taliban. That agreement said U.S. troops would be out of Afghanistan in 18 months, provided the Taliban honored a commitment to fight terrorist groups, with most attention seemingly focused on the Islamic State group’s affiliate in the country.

U.S. officials also say that the U.S. has critical classified equipment at bases in Afghanistan that needs to be moved out, which will take time. They also said that troop withdrawals from Afghanistan need to be coordinated with other coalition allies that also have forces there.