US-backed Kurdish and Arab fighters have opened up a new front against so-called Islamic State (IS) in northern Syria, close to the Turkish border.
The offensive by the Syrian Democratic Forces (SDF) is reported to be directed at Manbij, a town under IS control.
The UK-based Syrian Observatory for Human Rights said US-led pre-dawn air strikes on Manbij killed 15 civilians.
The operation aims to cut IS off from an area it uses to move weapons and fighters across the border.
Thousands of forces have been deployed in the offensive, which could take weeks, unnamed US officials told Reuters news agency.
The SDF fighters are being supported by US special forces.
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US officials say the SDF force is mostly Arab with a small number of Kurds.
However, the Syrian Observatory (SOHR) said most of the fighters were from the Kurdish Popular Protection Units (YPG) militia.
Turkey considers the YPG a terrorist group and fiercely opposes any Kurdish advances near the border.
The SOHR added the troops were about 15km (nine miles) from Manbij.
The clashes were “fierce and intense”, SDF military adviser Nasser Haj Mansour told AFP news agency.
The SDF has emerged as a key ally of the US-led anti-IS coalition over the past two years, leading the fight against the militants on the ground in northern Syria.
They began an offensive last week to expel IS from territory north of Raqqa, the de facto capital of the “caliphate” proclaimed by the group in 2014.
Crucial area: By Jonathan Marcus, BBC defence correspondent
Control of the Turkish-Syrian border is of crucial importance – US-backed Kurdish and Arab fighters are pushing towards the town of Manbij in an effort to deny IS a vital swathe of territory that offers a route for supplies and foreign fighters entering Syria.
Further to the west though towards Azaz, IS has itself gone onto the attack – a sign of the vital importance it attaches to this border zone.
It is here the complexities of the Syrian conflict are most apparent. US backing for Kurdish fighters – the most capable of the forces ranged against IS – outrages the Turkish government.
Turkish border towns have come under sporadic IS rocket or artillery fire while Turkey itself has been happy to mete out punishment to both Kurdish and IS fighters when the occasion demands.
Syria’s multiple conflicts are complicated enough but here they clash with the strategic interests of one of the country’s most powerful neighbours.