Baghdad -- The U.S. military has begun the process of withdrawing troops from Syria, a U.S. defense official confirmed to CBS News on Friday, saying it was expected to take between 90 and 120 days. The official told CBS News senior national security correspondent David Martin that equipment had begun moving out earlier this week.

America's operations against the Islamic State of Iraq and Syria (ISIS) in the country were to continue during the withdrawal process.

Col. Sean Ryan, spokesman for the coalition fighting ISIS, confirmed earlier in the day to The Associated Press that "the process of our deliberate withdrawal from Syria" was underway.

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"Out of concern for operational security, we will not discuss specific timelines, locations or troops movements," he said in a statement emailed to The Associated Press.

The Britain-based Syrian Observatory for Human Rights, which monitors the conflict in Syria through a network of activists on the ground, said the withdrawal began Thursday night. It said a convoy of about 10 armored vehicles, in addition to some trucks, pulled out from Syria's northeastern town of Rmeilan into Iraq.

Confusion over U.S. plans in Syria

Confirmation of the first withdrawals comes amid confusion over plans to implement President Donald Trump's pullout order and threats from Turkey to attack the Kurds, who have been America's partners on the ground in the war against ISIS in Syria.

There are 2,000 American troops in Syria. Mr. Trump's abrupt decision in December to pull them, declaring in a tweet the defeat of ISIS, sent shockwaves across the region and led to the resignation of U.S. Defense Minister James Mattis and the top U.S. envoy to the anti-ISIS coalition. It also led to major criticism that the U.S. was abandoning its local Kurdish allies amid Turkish threats of an imminent attack.

On Sunday, U.S. national security adviser John Bolton said American troops would not leave northeastern Syria until ISIS is defeated and the American-allied Kurdish fighters are protected. Secretary of State Mike Pompeo, who is on a tour of the region, has also sought to reassure the Kurds that they will be safe after U.S. troops withdraw from the country.

"These have been folks that have fought with us and it's important that we do everything we can to ensure that those folks that fought with us are protected," Pompeo said of the Kurds while visiting Erbil, the capital of Iraq's semi-autonomous Kurdistan region, after talks in Baghdad.

Giving a subsequent speech about broader U.S. policy in the Middle East at the American University in Cairo, Pompeo said "now is the time" to bring America's forces out of Syria. He stressed that the fight against ISIS would continue, but he did no repeat the need for the Kurds to be protected, or mention Bolton's previous vow to curb Iran's presence in Syria.

Russia's foreign ministry said Friday that it suspected the U.S. government was really intent on maintaining a military force inside Syria indefinitely, in spite of the pullout officially being announced by the Americans. Russia, the Syrian government's most vital ally, would be happy to see the U.S. pull out entirely of the war-torn country, where it has military bases and a significant troop presence.

Turkey's threat to Kurds could slow pullout

While the U.S. military pullout is now officially underway it is making a slow start.

The White House has not yet publicly altered it's most recent criteria, those announced by Bolton a week ago: that ISIS must be defeated and the Kurdish forces' security guaranteed before the American troops all come out. The first of those criteria is unmet, but feasible, as the terror group now holds only a small patch of land in Syria. The latter will no doubt prove the more difficult objective to meet.

Turkey considers America's Kurdish allies of the YPG militia -- who were vital in the ground war against ISIS -- a terrorist group. On Thursday, Turkey's foreign minister said President Recep Tayyip Erdogan had already made the decision to launch an offensive against the Kurds in northern Syria, and that it would go ahead regardless of the presence of American forces.

Foreign Minister Mevlut Cavusoglu went so far on Wednesday as to say that if the U.S. withdrawal from Syria was delayed, "then we will put this decision into practice," suggesting Turkey could go after the Kurds in short order.

The U.S. announcement that Mr. Trump's ordered withdrawal is underway might be enough to convince Turkey to hold fire against America's Kurdish battleground allies, for a while, but Erdogan still wants to go after them, and he could make good on his threats -- even on a small scale -- at any time.

The challenge for the United States will be to strike a deal with Turkey's government that ensures the YPG militia, that was so invaluable to Washington over the past handful of years, is protected when the withdrawal is complete. If not, the Trump administration would be accused of abandoning an ally to a grim fate.