NAIROBI (Reuters) - Turkish President Tayyip Erdogan said on Thursday a U.S.-backed offensive against Islamic State in northern Syria was largely being carried out by Arab rather than Kurdish fighters, in a sign of tacit approval for the operation near Turkey’s borders.

The Syria Democratic Forces (SDF) alliance, including the Kurdish YPG militia, thrust into Islamic State-held territory around the city of Manbij this week, backed by U.S.-led air strikes and U.S. special forces on the ground.

Turkey views the YPG as a terrorist organization and an extension of the Kurdistan Workers Party (PKK) militant group, which has waged a three-decade insurgency for Kurdish autonomy in Turkey’s own southeast. It resents U.S. support for the YPG.

Ankara, bent on preventing territorial gains by the Kurdish militia near its borders, has previously said it would not tolerate the YPG crossing west of the Euphrates River, into the area where the U.S.-backed operation is now underway.

“What we have been told is that the YPG will predominantly act as a logistical force here and the main force will be Arabs,” Erdogan told a news conference during a trip to Kenya.

“We are monitoring what is being done in this process through our own intelligence network and command chain. We will see,” he said, when asked about the YPG’s role.

Erdogan said that, based on discussions he had held with U.S. President Barack Obama about the offensive, the forces included around 2,500 Syrian Arab fighters and only 450 YPG members.

A Turkish military source said on Wednesday that Washington had informed Ankara of the operation but that Turkey would not play a direct role as it was beyond the range of Turkish artillery and Ankara would not back action involving the YPG.

Turkey, a member of the NATO military alliance, has been carrying out cross-border shelling of Islamic State positions in Syria in recent months but that action has been further west around the Syrian town of Azaz.

Turkey has shot back in response to rocket fire from Islamic State territory which has repeatedly hit the border town of Kilis. Military sources said five IS militants were killed and a weapons depot destroyed in the latest salvo on Wednesday.