The top U.S. military commander for the Middle East made a secret trip to northern Syria to meet a U.S.-backed alliance fighting the Islamic State group, the alliance's spokesman said, according to AFP.

General Joseph Votel, who heads U.S. Central Command (Centcom), met with leaders from the Syrian Democratic Forces in the first such trip under the new U.S. administration.

SDF spokesman Talal Sello told AFP that Votel "discussed the increase of coordination and support (to the SDF) in the era of Donald Trump".

"There were promises of heavy weapons in future stages," Sello said.

In an online statement, Sello said Votel had met with several SDF commanders.

"The results were positive. We discussed the developments in the Euphrates Rage campaign and shared military matters," Sello said.

He described the meeting as "confirmation of U.S. support for our forces".

An SDF source told AFP the visit lasted four hours.

According to a senior source in the SDF, "Votel confirmed the coalition's commitment to protecting Manbij from any attacks waged by Turkey or supported by it, as part of its previous commitment to protecting the area."

Manjib is a city in Aleppo province.

"We did not discuss opening corridors for Turkish-backed forces to enter areas under our control," the source said.

The Pentagon did not immediately respond to a request for comment.

Votel made a similar trip to Syria in May 2016, meeting with SDF commanders as well as U.S. military advisers working alongside them.

But this is his first trip under Trump's administration.

Founded in October 2015, the SDF is an alliance of Kurdish and Arab fighters that has seized swathes of territory from IS across northern Syria.

The United States has special operations forces advising the SDF on the ground in Syria, but no combat units.

On Wednesday, Votel told journalists travelling with him in the Middle East that more U.S. troops might be needed in Syria, although he stressed local forces would be the primary force.

"I am very concerned about maintaining momentum," Votel said, in comments reported by the New York Times and other outlets.

"It could be that we take on a larger burden ourselves."