US troop levels in Syria, which number around 1,000, have been under intense scrutiny since President Donald Trump last year ordered their complete withdrawal - only to later be convinced to leave some forces behind to ensure that Islamic State militants cannot stage a comeback.
The New York Times reported on Thursday that the Pentagon was preparing to send about 150 troops to conduct ground patrols with Turkish forces.
But Marine General Kenneth McKenzie, head of US Central Command, said the new mission would not require additional forces in the country.
"We're not going to increase our footprint on the ground in order to conduct those patrols," McKenzie said during a trip to Baghdad, while acknowledging that troop levels can fluctuate as forces rotate into and out of Syria.
"We're going to do patrolling with the Turks (and) we'll manage that within the current number while actively seeking opportunities to get smaller over time."
Turkey wants to clear its border of Syrian YPG militia, which it labels as a terrorist organization. But the YPG has been a main US ally in the fight against Islamic State in Syria, infuriating Turkey and straining ties with Washington.
As a way to address Turkish concerns, the United States has started carrying out the joint patrols with Ankara, with the first land patrol on Sunday and the fourth joint overflight of the area on Thursday.
But, at least so far, the joint patrols appear to have failed to satisfy demands from Ankara, which wants the operations to expand rapidly as far as 20 miles (32 km) from its border to create a safe zone controlled by Turkish troops.
On Tuesday, Turkey's Foreign Minister Mevlut Cavusoglu accused Washington of stalling and taking only "cosmetic steps."
Turkey and the United States have been at loggerheads over a host of issues in recent months, including Ankara's acquisition of Russian air defense systems. That led the Pentagon to kick the NATO ally out of a major US fighter jet program.
Turkey has relied heavily on President Tayyip Erdogan's personal ties with Trump to manage tensions. Erdogan and Trump are due to meet on the sidelines of the United Nations General Assembly later this month to discuss developments in Syria, trade and other bilateral issues.