Syria's Raqqa next in line for anti-IS campaign

French President Francois Hollande has warned that IS militants under attack by Iraqi and Kurdish forces in Mosul are already fleeing across the border to Raqqa.

"We can't afford mistakes in the pursuit of the terrorists who are already leaving Mosul for Raqqa," he said, pointing to the Syrian city as the next target.

On Friday, US Deputy Secretary of State Antony Blinken said the international coalition lined up against IS must now set its sights on Raqqa.

"We need to do both things, Mosul in Iraq and Raqqa in Syria," Blinken told radio station RTL in Paris, speaking in French.

"After Mosul, we'll look at Raqqa, absolutely. Raqqa is the most urgent, that's the town where Daesh [IS] plans its attacks overseas. Raqqa is the real capital," he added.

The 7,500-strong US-led coalition is carrying out airstrikes against IS in Iraq and Syria, as well as providing training, arms and equipment to local forces opposed to the militants.

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Raqqa on the Euphrates River, near the Turkish border and home to more than 200,000 people, in March 2013 became the first provincial capital in Syria to fall into the hands of rebels, at the time al-Nusra Front, then al-Qaeda's affiliate in the country.

But tensions soon erupted into clashes between al-Nusra and fellow jihadists of a precursor of the Islamic State group.

The latter imposed Islamic dress code in schools and attacked churches in Raqqa as they imposed a rule of terror marked by kidnappings and public beheadings.

Scene of worst atrocities

On January 6, 2014, all-out war broke out between the rival groups before IS's predecessor seized control of the whole city.

Five months later, Mosul fell to the militants and IS chief Abu Bakr al-Baghdadi, in a June 29 address proclaimed a "caliphate" straddling Iraq and Syria.

Raqqa remains the scene of IS's worst atrocities, such as the stoning to death of a woman accused of adultery and the killing of homosexuals by throwing them from rooftops.

In February 2015, IS posted an online video of the burning to death in a cage of a Jordanian pilot held in Raqqa province, after the militants seized the region's riches in wheat, cotton and oil resources as well as the regime's Tabqa air base where 170 soldiers were killed. 

In a rigid interpretation of Islamic laws, the jihadists in Raqqa have imposed strict bans on cigarettes as well as alcohol, while forcing men not to shave their beards and women to wear the "niqab" face veil.

Shops are also regulated, with only married couples allowed to go shopping together and mannequins in store fronts banned.

The main square in Raqqa is the venue for a trade in sex slaves, in particular kidnapped Yazidi girls, and where opponents are tortured to death.

Foreign jihadists have flowed from Turkey into Raqqa, 550 kilometres (340 miles) northeast of Damascus, and Western intelligence services say the city has served as the base for planning attacks abroad.