New threats raise the possibility of a clash between NATO allies.
There were no signs of further clashes between pro-Turkish forces and Kurdish militia groups that had controlled the city.
Votel said President Bashar al-Assad, with the backing of Russia and Iran, has essentially won the seven-year-old civil war.
U.S. Defense Secretary Jim Mattis on Sunday warned the Syrian government not to use chemical weapons in its civil war.
The two main rebel groups in the region have repeatedly denied negotiating with the Syrian regime.
Ankara's campaign against the Kurdish YPG is forcing the U.S. to pause its mop-up operations against ISIS.
Government forces effectively divided the enclave in two, further squeezing rebels civilians trapped inside.
The An-26, with 26 passengers and six crew aboard, crashed Tuesday just 500 meters from the runway.
The DoD says offensive operations against ISIS been paused because the Kurds have shifted to fight against Turkish forces.
he U.S. and Russia are clashing over an expert body to determine responsibility for chemical weapons attacks in Syria,.