AMMAN (Reuters) - A senior commander defected to Turkey on Wednesday from the U.S.-backed Syrian Democratic Forces (SDF), a leading combatant in the fight against Islamic State, Syrian rebel officials said, in the first such departure from its higher ranks.

They said Brigadier General Talal Silo handed himself in at dawn to the Free Syrian Army (FSA), an adversary of the SDF, near Jarablus city in northern Syria where he was then escorted to Turkey, which backs some FSA groups in that area.

The officials gave no reason for move by Silo, who was the SDF spokesman, but it follows months of growing discontent by some Arab tribes with the SDF, an alliance of Kurdish and Arab forces that is dominated by the Kurdish YPG.

Many local Arab tribes in areas controlled by the SDF complain they are marginalized in decision making and blame the YPG for discrimination against them, including the forced conscription of their youths. The YPG denies these allegation.

“Silo was secretly coordinating with commanders from the FSA and when he entered areas under their control he then crossed into Turkish territory,” said Ibrahim al-Idlibi, an FSA spokesman.

Kurdish fighters, alongside Arab allies, U.S. advisers and coalition jets, have driven Islamic State from swathes of territory including its former Syrian headquarters in Raqqa city.

The Kurdish YPG militia and its political allies have carved out autonomous cantons in the north, and now control nearly a quarter of Syria. Their influence angers neighbor Turkey, which considers the YPG an extension of the banned Kurdistan Workers’ Party that has fought a decades-long insurgency on Turkish soil.

The U.S.-led coalition said it was “aware of reports of Talal Silo’s apparent departure from the SDF, but have no further details on his current status at this time.”

Reuters could not immediately reach the SDF for comment.

U.S.-backed militias and the Syrian army have been advancing in separate offensives against Islamic State in eastern Syria, piling pressure on a small stretch of remaining territory the group still holds in oil-rich areas near the Iraqi border.