Russia's Ministry of Defense reported on Friday that pro-government forces circled around US military advisers in eastern Syria to reach the Iraqi border, dealing what could be a major blow to the declared USA mission to defeat Islamic State militants in the desert region.
"Our troops are advancing in Al Meshleb and control parts of it", SDF spokesman Talal Sello said.
SDF fighters were armed mostly with light weapons including Kalashnikovs, and were also returning mortar fire on IS positions.
The Britain-based group reported 23 civilians killed in the overnight strikes, including 15 who were in an internet cafe in the western suburb of Jazra, just outside the city.
Mahmoud said the US-led coalition against Isis was supporting the ground operation from the air, but that American special forces stationed in the area had not entered the city.
The evening news broadcast on Al-Manar TV, which belongs to the Lebanese militant group Hezbollah, hailed the advance of the pro-government forces, and the Hezbollah fighters embedded with them.
The Syrian Democratic Forces (SDF), a coalition of Kurdish and Arab fighters, breached the outer city limits of Raqqa, seizing a 1,000-year-old fortress in the west and a neighbourhood on the eastern side of the city, two days after announcing the start of the offensive.
"Hundreds" of U.S. military personnel are taking part in the Raqqa offensive, according to the Pentagon, which said Thursday it believed up to 2,500 extremists were still holed up in Raqqa.
An estimated 300,000 civilians were believed to have been living under IS rule in Raqa, including 80,000 displaced from other parts of Syria.
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The SDF has scored a series of victories since launching its operation to take Raqa in November, including capturing the strategic town of Tabqa and its adjacent dam in May.
But thousands have fled in recent months, and the United Nations humanitarian office said on Tuesday that it estimated about 160,000 people remained in the city.
On Tuesday, it made the long-awaited announcement that the battle for Raqa itself had begun.
That decrease could indicate IS intends to use remaining civilians "as human shields", the aid group said.
Along with Mosul in Iraq, Raqa was one of the twin pivots of the self-styled Islamic "caliphate" that IS declared almost three years ago.
He said that Syrian government troops had secured territorial gains in the southern Suwayda province near the Jordanian border, but encountered resistance from the USA -led coalition.
US defense officials said the weapon fired by the drone did not detonate and no one was hurt.
It was the third time the coalition has struck pro-regime forces near Al-Tanaf in less than a month.