Mosul/Damascus -  Dozens of dead bodies of civilians killed while fleeing an Islamic State-held neighbourhood in Mosul were lying on Saturday in a street close to the front line with Iraqi armed forces, a Reuters TV crew reported.

The bodies belonged to men, women and children. Bags in which they carried their belongings were strewn around the street leading out of the Zanjili districts, one of three still in the hands of Islamic State in Mosul. Hundreds others managed to reach the government lines, some wounded and some carrying apparently dead bodies in blankets.

ASSAD SAYS 'WORST BEHIND US'

Syria's President Bashar al-Assad has said the worst of his country's six-year war "is behind us" after a string of advances by his forces backed by key ally Russia.

In recent months, Assad's troops and allied fighters have retaken territory across the country, including recapturing all of Aleppo city in December after years of fighting.

"Things now are moving in the right direction, which is a better direction, because we are defeating the terrorists," he said in an interview in Damascus with the India-based WION television station. "Unless the West and other countries and their allies, their puppets, support those extremists in... a very massive way, I'm sure the worst is behind us."

The interview was released by the Syrian presidency on Saturday. More than 320,000 people have been killed in Syria since the conflict began in March 2011 with demonstrations against Assad's government.

The Syrian leader repeated previous denials of responsibility for an alleged chemical weapons attack in April on the rebel-held town of Khan Sheikhun.

And he said a peace initiative agreed by his allies Russia and Iran, and rebel backer Turkey, in the Kazakh capital Astana last month was a promising idea. "Until this moment we haven't had any real political initiative that could produce something, although Astana has achieved let's say partial results through the recent de-escalation areas in Syria, which was positive," he said.

Meanwhile, US-backed forces captured a town west of Raqqa from Islamic State on Friday as they push toward the militants' Syrian stronghold, a security source and a monitoring group said. The United States said this week it started distributing weapons to Kurdish YPG militiamen who form a key part of the Syrian Democratic Forces (SDF) alliance which Washington is backing in its fight against the militants.

"It's been around three days that the SDF has been attacking al-Mansoura. Today, from two sides, they entered, and it was liberated," a security source close to the SDF told Reuters.

The British-based Syrian Observatory for Human Rights said al-Mansoura was the largest town in the western Raqqa countryside. There were still some clashes taking place around pockets of IS resistance in the town, it said.

The SDF, which includes the YPG and Syrian Arab fighting groups, said last month it expected to storm Raqqa in early summer as part of a multi-phased offensive.

The alliance has advanced in recent months to within several kilometres (miles) of the centre of Raqqa, although their gains have taken longer than predicted with fierce resistance from IS.

Fighting since late last year has displaced tens of thousands of people according to United Nations sources, many of whom head for camps in the area.

The families of Islamic State fighters have fled the US-backed offensive, including for neighbouring Deir al-Zor province, another IS stronghold, the Observatory says.

The US-led coalition backs the SDF with air strikes and military advisors on the ground.