Its regional spokesman David Swanson said as many as 100,000 people "may stay trapped" inside during the assault.
He said SDF forces were also advancing on the northern front outside the city, and had repelled an attack by IS fighters as they pushed towards the city limits from the west.
Col. Joseph Scrocca, a spokesman for the USA coalition fighting ISIS, said in a statement sent to Reuters that the militants' resistance had been "minimal" outside Raqqa and they were retreating "to protect their fortifications inside the city".
The Russian military on Friday accused the U.S-led coalition in Syria of providing safe corridors for Daesh to leave the area around its stronghold of Raqqa, the Associated Press reported. "It raises a question (about) why they do it and what their real goals are".
The Observatory said the SDF now controlled around two-thirds of Al-Meshleb and was some 400 metres (yards) from the neighbouring Al-Senaa district.
ISIS extremists have been fighting back against the advancing forces with snipers as well as drones armed with explosives, according to the SDF.
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.
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Murray defeated 21-year-old Khachanov 6-3, 6-4, 6-4 in two hours and four minutes to register his 650th match win of his career. Murray played cleanly to beat 53rd-ranked Karen Khachanov of Russian Federation 6-3, 6-4, 6-4 on Court Philippe Chatrier .
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