WASHINGTON — In July, Turkey granted the United States access to use its military bases to conduct air operations against the Islamic State (IS) and to station search and rescue teams. This may have insulated it from Western frustration at Turkey’s slow efforts to close off a remaining 98-kilometer (60-mile) stretch of the Turkish-Syrian border that US officials say is being used by foreign fighters to enter Syria and by IS to smuggle oil. In addition, the United States recognizes that efforts to seal the Turkish-Syrian border have been further complicated by the lack of sufficient Syrian rebel allies to hold land on the Syrian side of the border, by Turkey’s rejection of an autonomous Syrian Kurdish entity emerging on its border and by the humanitarian need to let Syrian refugees flee from the conflict to safety, according to diplomats and analysts.

“I have had repeated conversations with [Turkish] President [Recep Tayyip] Erdogan about the need to close the border between Turkey and Syria,” US President Barack Obama said after meeting Erdogan in Paris on Dec. 1. “We’ve seen some serious progress on that front, but there are still some gaps.”

“There’s about 98 kilometers that are still used as a transit point for foreign fighters, [IS] shipping out fuel for sale that helps finance their terrorist activities,” Obama said. “So we have been having our militaries work together to determine how a combination of air and Turkish ground forces on the Turkish side of the border can do a much better job of sealing the border than currently is. And I think President Erdogan recognizes that.”

While Turkey and the United States have different priorities in Syria, they functionally cooperate, Turkish analyst Henri Barkey said.