US-backed force 'mopping up' last IS holdouts in Raqqa

Syrian fighters backed by US special forces battled to clear the last remaining Islamic State group militants holed up in their crumbling stronghold of Raqqa on Thursday.

Most of Raqqa, long a byword for the militants' most gruesome atrocities, is now in the hands of US-backed fighters supported by waves of heavy airstrikes by a military coalition led by Washington.

"The Syrian Democratic Forces and American special forces began a mopping up operation in Raqqa," the Syrian Observatory for Human Rights said on Thursday.

The Britain-based monitor said militant holdouts were still hiding in underground shelters in a part of the city centre where a football stadium and former government buildings are located.

But the operation was being slowed down by large numbers of mines planted by the militants in the city, where they have been under siege for three months, it said.

The extremists seized Raqqa in early 2014, making it their de facto Syria capital. They are thought to have used the city to plan attacks abroad.

On Wednesday the SDF said they were in the "final stages" of capturing Raqqa as the Observatory said the US-backed fighters controlled 90 percent of the northern city.

The US-led coalition supporting the SDF estimated that 65-70 percent of Raqqa was under the control of the alliance of Kurdish and Arab fighters.

'Trapped civilians' 

Meanwhile, tens of thousands of civilians have fled the Raqqa fighting in recent months but thousands are still trapped inside the city according to the UN's Office for the Coordination of Humanitarian Affairs (OCHA).

"We now estimate that up to 15,000 civilians remain trapped in Raqqa city, although exact figures remain difficult to verify due to the situation on the ground," OCHA's Linda Tom told AFP.

She said the civilians, many of them women and children, "are facing incredibly difficult conditions", including food, water and medical shortages.

IS also holds pockets of territory elsewhere in Syria, notably in eastern parts of the central provinces of Homs and Hama, but it has come under attack by Russian-backed government forces there too.

The militant group has seen the territory under its control fast diminish in recent months in the face of multiple offensives against its fighters in both Iraq and Syria.