US-backed rebels announce start of operations to capture Raqa
BEIRUT - The Syrian Democratic Forces alliance that on Sunday announced the start of operations to capture Raqa from the Islamic State group was formed just over a year ago.
It has since emerged as a key fighting force against ISIS in northern Syria and the main ground partner of the US-led coalition that launched a campaign against the jihadist group in mid-2014.
The SDF has scored a series of victories in the past 12 months, the most important the recapture in August of the strategic northern city of Manbij.
But the dominant role of Kurdish forces in the alliance has raised concerns with Turkey, whose military has hit SDF positions.
The SDF was formed in mid-October last year as Kurds, Arab Muslims and Christians, and other groups joined forces to battle IS in northern Syria.
Syria's five-year civil war has seen the country divided into a patchwork of fiefdoms but in IS the disparate members of the SDF found a common enemy.
The alliance is estimated to command about 30,000 fighters, including some 25,000 Kurds and 5,000 Arabs.
The Syrian Kurdish People's Protection Units (YPG), which had already dealt ISIS several defeats including seizing the key border town of Tal Abyad, forms the backbone of the alliance.
Along with the Kurdish female Women's Protection Units, the SDF includes Arab factions, Syriac Christian fighters and Turkmen units.
The Raqa Falcons Brigade, a 1,000-strong Arab force whose fighters all hail from Raqa, is expected to be a key component of the fight for the city.
After launching the coalition air war against ISIS in Syria and Iraq, Washington struggled to find a reliable partner on the ground.
A much-touted $500 million programme to build a rebel army to fight ISIS collapsed after many candidates failed to pass the screening process, and one group surrendered equipment to an Al-Qaeda affiliate.
But since its creation the SDF has benefitted from strong US backing, including weapons drops and air strikes in support of its operations.
Shortly after the SDF was formed, the White House announced the first sustained deployment of US special forces to Syria, reversing a longstanding refusal to put boots on the ground.
Some 50 special operations personnel were deployed in northern Syria and the number has now grown to between 200 and 250.
US officials including President Barack Obama's envoy to the coalition Brett McGurk and Joseph Votel, the head of Central Command, have since met with senior SDF commanders on visits to northern Syria.
The last months of 2015 saw a string of early victories for the SDF, with the alliance clearing ISIS from some 200 villages in the northeastern province of Hasakeh.
The SDF also moved against ISIS in Aleppo province, where its fighters captured the Tishreen Dam, and fought off a jihadist offensive against the SDF-held town of Ain Issa, some 50 kilometres (30 miles) from Raqa.
SDF fighters clashed with other, mostly Islamist, rebel forces in Aleppo province, seizing control of the town of Tal Rifaat and the Minnigh airbase.
In February the SDF took Al-Shadadi, which had been the largest town controlled by ISIS in Hasakeh, and the Kibabeh oil field to the northeast.
In June the alliance launched its most ambitious offensive yet, to take the city of Manbij just south of the Turkish border and sever a key supply route to Raqa.
The alliance faced fierce resistance, including near-daily suicide bombings, but was able to seize control of Manbij in early August.
The rise of the SDF has raised deep concerns in Turkey, which considers the YPG a "terrorist" offshoot of the Kurdistan Workers' Party (PKK) that has been waging an insurgency inside Turkey since 1984.
Turkey launched an operation inside Syria on August 24 alongside allied opposition forces who have managed to retake the IS stronghold of Jarabulus and the symbolically important town of Dabiq.
But one of the operation's goals is also to check the advance of Syria's Kurds, and Turkish forces have carried out air strikes against YPG positions.