ANKARA (Reuters) - Some militants have withdrawn from the demilitarization zone in Idlib following a deal between Russia and Turkey on the northwestern Syrian region, a U.S. official said on Wednesday.

Idlib and adjacent areas are the last stronghold of rebels who rose up against President Bashar al-Assad in 2011. The area is also home to an estimated 3 million people, more than half of whom have already been displaced at least once during the war.

Turkey and Russia reached a deal to set up a buffer zone running 15-20 km (9-13 miles) deep into rebel territory that had to be evacuated of all heavy weapons and all jihadists by Monday. Hayat Tahrir al-Sham (HTS), a jihadist alliance spearheaded by al-Qaeda’s former Syrian affiliate, is the most powerful jihadist alliance in the region.

“The withdrawal of heavy weapons is complete by all accounts,” James Jeffrey, the U.S. special representative for Syria engagement, told reporters in Ankara during a brief visit with the U.S. secretary of state. “There is some question as to whether everybody from (Hayat Tahrir al-Sham) has left.”

He said 50,000-70,000 remained in the area.

“Most of them are part of the opposition that we used to work with and the Turks still do,” he said. He estimated that between 7,000-10,000 are militants.

Approximately 40 percent of the country does not fall under control of Assad’s government, he added.

Turkey and the United States hold regular coordinated patrols in northern Syria’s Manbij area, where the Kurdish YPG militia has a presence. Turkish President Tayyip Erdogan has warned that Ankara “will do what is necessary” to ensure the YPG leaves Manbij as per the agreement with the United States.

Turkey considers the YPG a terrorist organization.

Erdogan told U.S. Secretary of State Mike Pompeo that Turkey could easily clear the town if the United States failed to do so, Turkey’s foreign minister Mevlut Cavusoglu said on Wednesday.

Jeffrey said the negotiations between Pompeo and Cavusoglu are now being carried out and that the United States has troops in Turkey training with Turkish troops to carry out joint patrols around Manbij.