US-backed Kurdish and Arab fighters have opened a new front against the Islamic State group in northern Syria, thrusting into a strategic jihadist pocket along the Turkish border, a monitor said on Wednesday.
The swathe of territory controlled by ISIS on either side of the Euphrates River has long been a key target for Washington as it is seen as the main entry point for foreign fighters.
The Pentagon has deployed more than 200 special forces troops alongside the Syrian Democratic Forces (SDF), a Kurdish-led alliance in which it has been trying to boost the Arab element.
US-led coalition aircraft carried out intense air strikes on the IS-held town of Manbij, 30 kilometers (20 miles) west of the Euphrates, in support of the offensive, the Syrian Observatory for Human Rights said.
The Britain-based monitoring group, which relies on reports from medics and activists on the ground, said three children were among 15 civilians killed in the pre-dawn raids.
"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 18 kilometers (11 miles) of Manbij.
The offensive is one of two the SDF has launched against ISIS in northern Syria in recent weeks.
Washington's support for the SDF has angered NATO ally Turkey as its most powerful component is the Kurdish People's Protection Units (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.
But in an apparent attempt to appease Turkey, US sources have claimed YPG fighters will only comprise a minority of the forces advancing on Manbij, and that all Kurdish fighters will withdraw to allow Arab fighters to take control of the city.
AFP contributed this report.