US-backed Kurdish and Arab fighters launch new Syria offensive to cut off IS

US-backed Kurdish and Arab fighters have opened a new front against the Islamic State (IS) group in northern Syria, thrusting into a strategic militant pocket along the Turkish border, a monitor and a US official said on Wednesday.

The IS-controlled territory on either side of the Euphrates River - known as the Manbij pocket - has long been a key target for Washington as it is seen as the main entry point for foreign fighters joining the militant group.

A US military official told Reuters that the area is the group's "last remaining funnel" to Europe.

The Pentagon has deployed 200 special forces troops, in addition to 50 already on the ground, alongside the Syrian Democratic Forces (SDF), a Kurdish-led alliance including Arab fighters.

The official said that the forces will act as advisors and will not engage in direct combat, staying away from the front lines.

US-led coalition warplanes carried out intense air strikes on the IS-held town of Manbij, 30km west of the Euphrates, in support of the offensive, the Syrian Observatory for Human Rights said.

"The campaign for Manbij began on Tuesday," Observatory chief Rami Abdel Rahman told AFP. "Over the past 24 hours, the SDF have seized control of nine villages... west of the Euphrates," he said.

The advance has brought SDF forces within 18km of Manbij.

The offensive is one of two the SDF has launched against IS in northern Syria in recent weeks.

Last month, the alliance launched an assault on the militants north of their de facto Syria capital Raqqa, seizing dozens of villages in the north of Raqqa province.

Washington's support for the SDF - and photos that emerged of US commandos wearing insignias of the Kurdish People's Protection Units (YPG) - has angered NATO ally Turkey as its most powerful component is the YPG.

Ankara regards the YPG as a branch of the rebel Kurdistan Worker's Party (PKK), which has fought a three decade insurgency against the Turkish state.