WASHINGTON, Dec. 23 (Xinhua) -- U.S. President Donald Trump said on his Twitter on Sunday that he had a phone conversation with Turkish President Recep Tayyip Erdogan over Syria issues.

"We discussed ISIS (IS), our mutual involvement in Syria, and the slow and highly coordinated pullout of U.S. troops from the area," said Trump on his Twitter account, describing the phone conversation as "long and productive."

The two leaders also discussed expanded trade between the two countries in their phone call.

Trump agreed to withdraw U.S. troops from Syria during his phone conversation with Erdogan on Dec. 14, the Associated Press cited two officials in a report.

The State Department cleared Turkey to purchase a package of Patriot missile systems on Dec. 18, with an estimated cost of 3.5 billion U.S. dollars.

The Trump administration announced on Dec. 19 that it started returning U.S. troops home from Syria after claiming a victory in the fight against the IS, without revealing any detailed timetable.

The decision to withdraw U.S. troops in Syria was widely regarded as the last straw for Defense Secretary James Mattis' resignation on Dec. 20.

Following Mattis, Brett McGurk, the U.S. envoy for the global coalition to defeat Islamic State (IS), also resigned on Dec. 21, which was another sign of senior officials' objection over the U.S. troops withdrawal from Syria.

The time-frame for U.S. troops withdrawal from war-torn Syria is expected to be 60 days and 100 days, Reuters reported, citing an anonymous administration official earlier in the past week.

The United States has deployed troops to Syria fighting the IS since 2015, and there are more than 2,000 U.S. soldiers in Syria.