BEIRUT, Sept 16 (Reuters) - A small number of U.S. forces entered the Syrian town of al-Rai near the Turkish border on Friday as part of operations to coordinate air strikes against Islamic State, a senior rebel source said.

However, the five or six U.S. military personnel were then forced to withdraw towards the Turkish border after Syrian rebels protested against their presence in the town, the source said.

A monitoring group, the Syrian Observatory for Human Rights, also reported the incident and said the U.S. forces had left al-Rai but were still on Syrian soil.

One U.S. official, speaking on condition of anonymity, said initial reports appeared to confirm the incident involving a small group of U.S. forces.

The Pentagon did not immediately respond to requests for comment on the incident. But it did acknowledge that U.S. special operations forces are accompanying Turkish and vetted Syrian opposition forces battling Islamic State in and around the area of the Syrian border near al-Rai and the town of Jarablus, further east.

Turkeys military said in a statement U.S. special forces were supporting an operation being carried out in northern Syria.

Turkish-backed rebels have been battling Islamic State militants along the border as part of operation Euphrates Shield, which was launched last month, and in recent weeks pushed the jihadists away from the frontier with the support of Turkish warplanes and tanks.

The fighters Ankara supports were pushing south on Friday and five rebels and five IS militants had been killed in the region, the Turkish army said.

The rebel source said the U.S. forces had entered al-Rai, 2 km (1 mile) inside Syria, as part of that operation.

In a video circulated on the internet purportedly showing the incident, fighters in al-Rai chanted anti-U.S. slogans and threatened violence against them as a number of vehicles drove out of the area.

The Turkish operation also aims to push U.S.-backed Kurdish forces, which have separately been fighting Islamic State, away from the border.

Clashes between Turkish-allied forces and the Kurdish YPG militia, a key U.S. partner in the fight against Islamic State, have caused tension between Washington and Ankara.

(Reporting by John Davison and Tom Perry in Beirut and Phil Stewart in Washington; Editing by Dominic Evans)