US-backed fighters pierced jihadist-held Raqa from the south for the first time on Sunday, crossing the Euphrates River to enter a new part of the Syrian city, a monitor said.

The Syrian Democratic Forces have spent months closing in on the Islamic State group's bastion Raqa and entered the city's east and west for the first time last month.

On Thursday, the US-backed Arab-Kurd alliance sealed off the jihadists' last escape route by capturing territory on the southern bank of the Euphrates.

"Today, they entered Raqa's south for the first time and seized the Al-Hal market," Rami Abdel Rahman, head of the Syrian Observatory for Human Rights, said on Sunday.

He said some SDF fighters had advanced north across the Euphrates River, while others had attacked Al-Hal from the adjacent district of Al-Meshleb in Raqa's east.

"The market is fully under SDF control but IS is waging a counter-attack," Abdel Rahman said.

The SDF's Operation Wrath of the Euphrates announced it had captured the Al-Hal market on Sunday.

Abdel Rahman also said 11 civilians, including four women and five children, were killed in coalition air strikes on the western Raqa district of Al-Daraiya late Sunday.

The new deaths put at more than 200 the civilian toll from coalition raids on Raqa since the US-backed SDF entered the city on June 6. 

The Observatory said "dozens" of SDF fighters had been killed in Raqa in the same period, "including 36 in the past week". 

The SDF have since seized a handful of neighbourhoods in the east and west but are facing fierce resistance by IS as they push closer to the city centre.

- 'Destroying tunnels' -

SDF fighters were battling IS on Sunday inside the eastern district of Al-Senaa, which the jihadists retook after an initial advance by the US-backed forces last month. 

Al-Senaa is key for both the SDF and IS because it is adjacent to the city centre, where most IS fighters defending Raqa are thought to be holed up.

IS pushed the SDF out of Al-Senaa on Friday, using dozens of jihadists disguised in SDF uniforms as well as a slew of car bombs. 

After two days of a counter-offensive, the SDF had retaken about 70 percent of it by Sunday, the Observatory said. 

"Our forces are about 100 metres (yards) from Baghdad Gate," said Syrian Elite Forces spokesman Mohammad Khaled Shaker, referring to the entrance to Raqa's Old City. 

"We are combing the area and destroying tunnels to prevent any new infiltrations towards our positions," Shaker told AFP. 

According to the coalition, an estimated 2,500 IS jihadists are defending the northern city.

IS overran Raqa in 2014, transforming it into the de facto Syrian capital of its self-declared "caliphate," which it declared three years ago.

The city became infamous as the scene of some of the group's worst atrocities, including public beheadings, and is thought to have been a hub for planning attacks overseas.

The United Nations has expressed concern for up to 100,000 civilians it says are still trapped in the city.

Source: AFP