BEIRUT (Reuters) - U.S.-backed Syrian militias said on Tuesday they had launched a battle to free Raqqa, Islamic State's base of operations in Syria, attacking from the east, west and north of the city.
The Raqqa assault will pile more pressure on the jihadist group's self-declared caliphate as it faces defeat in the Iraqi city of Mosul and retreats across much of Syria.
Here are some facts about Raqqa:
Raqqa is Syria's sixth-largest city and sits on the River Euphrates around 90 km (56 miles) from the Turkish border in north central Syria.
It is 40 km downstream of the Tabqa dam, Syria's largest, behind which stretches Syria's largest lake, Lake Assad.
The hardline Sunni militant group overran Raqqa in January 2014, seizing control from rebel groups opposed to the rule of Syrian President Bashar al-Assad.
The United Nations said in March the city contains around 200,000 people, just under its pre-war population. The Britain-based Syrian Observatory for Human Rights war monitor estimates there are at least 150,000 civilians still there.
According to the U.S.-led coalition, 3,000-4,000 Islamic State fighters could be holed up in Raqqa city, erecting defences against the anticipated assault.
The coalition says it is careful to avoid civilian casualties in its bombing runs in Syria and Iraq but the U.N. human rights office has raised concerns about increasing reports of civilian deaths.
Islamic State militants have reportedly forbidden civilians from leaving, it said.
Islamic State has imposed its very strict interpretation of Islamic law on Raqqa's residents, carrying out regular public executions, lashings and violent punishments for infringements of their rule.
Tens of thousands of people have been displaced by fighting around Raqqa since late last year.
The Syrian Democratic Forces (SDF), an alliance of Kurdish and Arab militias, have been closing in on Raqqa since launching their campaign to take the city in November 2016.
The United States-led coalition against Islamic State supports the SDF with air strikes and special forces on the ground.
For months, the SDF has encircled Raqqa from the north, east and west, facing tough resistance from the jihadists who have used the city as a hub for planning attacks abroad.
Now Islamic State's only means of crossing to its main territory south of the River Euphrates is by boat, after air strikes knocked out several bridges.
The Raqqa campaign has been the source of tension in ties between the United States and NATO-ally Turkey. Possible Kurdish influence in the future running of the predominantly Arab city is sensitive both for residents and for Turkey.
The powerful Kurdish YPG militia is the SDF's key component and has become the main U.S. partner in the fight against Islamic State in northern Syria.
Turkey views the YPG as the Syrian extension of the outlawed Kurdistan Workers Party (PKK), which has waged a three-decade insurgency within Turkey, and fears growing Kurdish ascendancy along its border. Ankara raised fierce objections last week when the United States said it started supplying arms to the YPG for the Raqqa assault.
The SDF has said it will hand control of Raqqa to a civilian council from the city once it has defeated Islamic State, echoing the pattern in other areas captured by the SDF from the militants.
Islamic State controls swathes of Syria's eastern deserts bordering Iraq and most of Deir al-Zor province, which would be its last major foothold in Syria after losing Raqqa.
The militants have made enemies of all sides involved in the six-year Syrian conflict, losing territory to various separate military campaigns over the past year.
A modern-day provincial transport hub and market town, Raqqa city was built by the Abbasid Islamic Caliphate in the eighth century, serving as its capital at one point.
Raqqa has been inhabited since antiquity and contains many archeological and architectural sites of interest. The United Nations has said sites in and around the city have been extensively looted during the war and religious buildings have been damaged.
Islamic State released a video of them bombing a large part of the Uwais al-Qarani shrine complex in March 2014.
Reporting by Ellen Francis and Lisa Barrington; Editing by Raissa Kasolowsky