Five facts on the jihadist-held Syrian city of Raqa

Islamic State jihadists established a caliphate in 2014 after seizing a large swathe of territory in Iraq and Syria, but they have now lost over 50% of the territory they once controlled (AFP Photo/) (Welayat Raqa/AFP/File)

Beirut (AFP) - As a US-backed alliance of Kurdish-Arab forces launches an assault to capture Raqa from the Islamic State group, here are five things to know about the northern Syrian city:

- Ancient capital -

In an area inhabited since antiquity, Raqa reached its heights under the Abbasid caliphate. From 796 to 809, the powerful caliph Harun al-Rashid transferred the capital of his empire from Baghdad to Raqa, which sat at the crossroads of key trade routes. Major works were ordered and the city was dotted with grand palaces and mosques.

The caliph's court returned to Baghdad in 809 and Raqa remained a major administrative centre for the western part of the empire. In 1258 the city was destroyed by the Mongol invasion.

- Strategic location -

Raqa and its eponymous province occupy a strategic location where several major roads intersect on the banks of the Euphrates River in northeastern Syria.

It is east of Syria's second city Aleppo, just 90 kilometres (55 miles) south of the Turkish border, and less than 200 kilometres (125 miles) from neighbouring Iraq.

Raqa previously prospered from agriculture in the fertile river valley and benefitted from nearby hydroelectric dams that generated power for much of Syria.

- First major city to fall -

After the 2011 start of Syria's civil war, Raqa in March 2013 became the first provincial capital to fall into the hands of rebels, at the time the Al-Nusra Front, Al-Qaeda's then-affiliate in the country.

But tensions soon erupted into clashes between Al-Nusra and fellow jihadists of a precursor of IS.

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 in Iraq fell to the jihadists and IS chief Abu Bakr al-Baghdadi, in a June 29 address at a mosque in Iraq's second largest city, proclaimed a "caliphate" straddling Iraq and Syria.

- Key IS bastion -

Raqa became a key city in IS's self-declared "state", a hub for the organisation of its activities and attacks in Syria, Iraq and further afield.

IS took over all levels of civil administration, rewriting school curriculums, establishing Islamic courts and creating police units to implement Islamic law.

Raqa became the scene of some of IS's worst atrocities, including gruesome executions, public displays of bodies and the selling of sex slaves in its central market.

- Long coalition target -

The city has long been an end target of the US-led coalition that launched a military campaign against IS in Syria and Iraq in mid-2014.

IS has suffered a string of territorial defeats in recent months at the hands of coalition-backed Iraqi forces and Syrian rebels.

Troops in Iraq launched a major offensive on Mosul last month and US Defence Secretary Ashton Carter said after that a simultaneous assault on Raqa would take place.

The SDF offensive is expected to begin with a "shaping operation", with ground troops surrounding Raqa and cutting off supply lines, backed by intensified coalition air strikes.