In a bloody counter attack, Islamic State militants have killed dozens of Syrian regime fighters. Reports from the Syrian Observatory for Human Rights said the strike by the extremists was fast and effective. "Isil has managed to drive out regime troops from the administrative borders of Raqqa province after a fierce counter-offensive," said the UK-based Observatory.

It said Isis (Daesh) had sent hundreds of reinforcements from their stronghold capital of Raqqa city to defend Tabqa, which also has an air base, about 35 miles to the west. Around 300 militants were seen with heavy machine guns and vehicles driving towards the area.

"More than 40 members of the pro-regime forces were killed," the monitoring group added of the bloody skirmishes mounted on Sunday 20 June.

Islamic State is continuing its offensive in Raqqa province despite a heavy onslaught by government troops, supported by Russian air strikes. Attacks by Isil on Sunday secured positions in the southwest of Tabqa and a Telegraph report claims the militants recaptured two villages from US-backed troops.

The US-led coalition backing the SDF carried out a barrage of air strikes on Monday to defend the villages, said Syrian Observatory head Rami Abdel Rahman.

Syrian government and Russian jets have stepped up the bombardment of a town in northern Syria held by IS. At least 18 air strikes by warplanes were carried out on Sunday, targetting Tabqa, to the west of Raqqa, say the Syrian Observatory. At least six people died in the bombing reported activist group Raqqa Is Being Slaughtered Silently.

In other areas, ousting IS has proved far from over. The US-backed Syrian Democratic Forces (SDF), a Kurdish-Arab alliance said they had surrounded the town of Manbij almost two weeks ago. But IS has hampered their claim of wresting control with suicide bombings which impede their progress of moving safely around the city

At least 28 SDF fighters were killed in an Islamic State attack on the Manbij area, reported the Syrian Observatory on Monday.

Rudaw reported that IS militants had regained control of three villages south of Manbij in the surprise offensive. But an SDF spokesman told Reuters that they were able to repulse the attack. "The situation is under control," said Sharfan Darwish, spokesperson for the Manbij Military Council. "They have many bodies on the ground."

"We are at the four gates to the city," he continued. "The whole city is booby-trapped. After 20 days of the campaign, we have yet to storm the city."