Washington brokered an initial ceasefire earlier this month after fighting that broke out as rebel groups advanced on Damascus and overthrew the rule of Bashar al-Assad. But on Dec. 19, a Turkish defense ministry official said there was no talk of a ceasefire deal between Ankara and the SDF.

"The ceasefire is holding in that northern part of Syria," Pentagon spokesperson Sabrina Singh told reporters.

The SDF is the main ally in a U.S. coalition against Islamic State militants in Syria. It is spearheaded by the YPG militia, a group that Ankara sees as an extension of Kurdistan Workers Party (PKK) militants that it outlaws and who have fought the Turkish state for 40 years.

The U.S. and Turkey's Western allies list the PKK as terrorist, but not the YPG and the SDF.

The United States has about 2,000 U.S. troops in Syria that have been working with the SDF to fight Islamic State militants and prevent a resurgence of the group, which in 2014 seized large swathes of Iraq and Syria but was later pushed back.