Raqqa offensive begins: US-backed rebel forces launch bid to capture Isis's Syrian stronghold

US-backed rebel forces on the ground in Syria have launched an offensive to capture Isis's main stronghold of Raqqa, according to a statement.

The Kurdish-led Syria Democratic Forces (SDF) announced the start of the battle to take Raqqa in a news conference on Sunday, exactly three weeks after the start of a similar offensive against Mosul, the largest Isis-held city in Iraq.

"The major battle to liberate Raqaa and its surroundings has begun," said Jihan Sheikh Ahmed, a spokeswoman for the SDF, at the conference in Ein Issa, some 50km (30 miles) north of Raqqa itself.

The US sees the SDF as representing the most effective rebel fighting force in Syria, but it is dominated by the Kurdish People's Protection Units, or YPG.

That has complicated planning for the operation to liberate Raqqa, with Turkey wanting a role but viewing the YPG as a terrorist organisation. Similar challenges exist over Turkey's role in Mosul.

On Sunday, SDF fighters said a US-brokered deal had ensured Turkey would not be involved in the Raqqa operation.

"We have agreed definitively with the (US-led) international coalition that there will be no role for Turkey or the armed factions allied with it in the operation," Talal Sello of the SDF told the AFP news agency.

He said the SDF had received new weapons from the US-led coalition to use in the battle, including anti-tank missiles, and that the offensive would involve 30,000 troops.

Fighting had already begun on Saturday night, he said, adding that it would involve two stages: "first liberating the countryside around Raqqa and isolating the city, and second taking control of the city."

"The fight will not be easy, and will require accurate and careful operations because IS will defend its bastion knowing that the loss of Raqa will mean it is finished in Syria," he told AFP.

In Mosul, the army's progress has slowed after a rapid start to the assault in the second largest city in Iraq.

Iraq's special forces worked on Sunday to clear neighborhoods on Mosul's eastern edge of Isis militants, as the group launched what were believed to be retaliatory bombings elsewhere across the country, killing at least 20 people.

Though half a million people fled when Isis captured Mosul and declared its so-called caliphate in 2014, more than a million civilians are still thought to inhabit the city.

The current phase and slower pace highlight the challenges ahead for Iraqi forces as they press into more populated areas deeper inside Mosul, where the civilian presence means they won't be able to rely as much on airstrikes.

"There are a lot of civilians and we are trying to protect them," said Lt Col Muhanad al-Timimi. "This is one of the hardest battles that we've faced till now."