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An alliance of Kurdish and Arab fighters taking on the Islamic State group in Syria say the extremists will soon be defeated in the city of Raqqa.

Syrian Democratic Forces [SDF] spokeswoman Cihan Shekh Ehmed said on Sunday that after towns around Raqqa have been recaptured IS will be on its last legs.

"IS is currently in the throes of death," Ehmed Kurdish news agency ANF.

"We have liberated an area of thousands of kilometres squared in east, west and north Raqqa… [the city's] liberation will not take a long period of time."

Ehmed explained that the extremist group has been cut off from all supply routes to the city, which has been the Syrian heart of IS' self-styled "caliphate" since 2014.

"IS cannot reinforce its forces, ammunition and logistics. They are besieged in Raqqa from four sides," she said.

She added that IS was resorting to "savage methods" such as human shields to hold on the nearby town of Tabqa, which sits on a key supply route about 55 kilometres west of Raqqa.

SDF forces entered the key IS-held held town of Tabqa on Monday.

In an online statement, the SDF said it had captured IS-held positions in west Tabqa, including a roundabout, and part of a southern district.

On Monday, they entered it for the first time, the opposition-aligned Syrian Observatory for Human Rights said.

"They seized control of several points in the town's south and were advancing on its western edges," said Observatory head Rami Abdel Rahman.

He said US-led coalition warplanes were carrying out "intense" strikes in support of the offensive, but that one raid had killed three women and five children trying to flee Tabqa.

The US-backed group have set their sights on Tabqa and the adjacent dam as part of their broader offensive for the city of Raqqa.

Supported by US-led coalition air raids and special forces advisers, the SDF surrounded Tabqa in early April.