AMMAN, Jordan — More American troops may be needed in Syria to speed the campaign against the Islamic State, the top United States commander for the Middle East said on Wednesday.

“I am very concerned about maintaining momentum,” Gen. Joseph L. Votel, the head of the United States Central Command, told reporters accompanying him on a trip to the region.

“It could be that we take on a larger burden ourselves,” he added. “That’s an option.”

The current American strategy is to press the Islamic State from multiple directions by moving ahead with the offensive to retake the Syrian city of Raqqa even as Iraqi forces carry on their operation to take western Mosul.

Syrian Kurdish and Arab fighters backed by the United States are to play the principal role in seizing Raqqa, the de facto capital of the Islamic State’s professed caliphate.