— Reporting by CBS News' Sara Cook and Kathryn Watson

Roughly 400 troops — about 200 on a peacekeeping mission and about 200 in the northeast region of Syria — will remain there after the U.S. withdraws its troops, a senior administration official confirmed to CBS News Friday.

The senior administration official told CBS News the peacekeeping troops are trying to secure peace by definition, but described them as military troops who are observers monitoring the situation. They will be embedded with European allies. It's unclear how many European allies there will be. In addition to those 200 peacekeeping troops, forces currently at Al-Tanf Garrison in south Syria will also remain.

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White House press secretary Sarah Sanders had initially said Thursday night 200 peacekeepers would remain, without mentioning the roughly 200 more troops who will also remain. That number was clarified Friday.

The timeline for withdrawing troops, however, remains unclear, and the Trump administration has yet to announce the total defeat of the caliphate in Syria, as President Trump said would happen.

Sen. Lindsey Graham, R-South Carolina, who told CBS News' Margaret Brennan on "Face the Nation" earlier this week that the "all the work is not yet done" in Syria, issued a statement applauding Mr. Trump. "With his decision, President Trump has decided to follow sound military advice," Graham said.

The White House statement on the number of peacekeepers remaining came soon after President Trump spoke by phone with Turkish President Recep Tayyip Erdoğan. The two discussed Syria and agreed to continue coordinating on the creation of a potential safe zone.

Reuters first reported the rough number of peacekeepers who would remain.

U.S.-backed forces have closed in on the last sliver of ISIS territory in Syria. The U.S. has had a force of roughly 2,000 troops there to support local fighters and direct U.S. air power against ISIS targets.

Mr. Trump declared the defeat of ISIS in a tweet in December.

"We have defeated ISIS in Syria, my only reason for being there during the Trump Presidency," the president tweeted in late December.