Jihadists of the Islamic State group drove Syrian regime troops out of Raqa province on Monday, killing dozens of fighters in a lightning counter-attack, a monitoring group said.
The attack was mounted late on Sunday in response to a regime offensive in the IS stronghold of Raqa launched on June 3 that advanced about 20 kilometres (12 miles) toward the town of Tabqa, said the Syrian Observatory for Human Rights.
It came as US Secretary of State John Kerry said he had read a memo from a group of frustrated diplomats urging strikes against the Syrian regime and found it "very good".
The "dissent cable" became public last week after 51 serving US officials signed a call for direct US military action to force President Bashar al-Assad`s regime to negotiate for peace.
The memo was seen as a criticism of President Barack Obama`s cautious approach, but the "dissent channel" is an approved mechanism for diplomats opposed to official policy.
Syria`s civil war began with the brutal repression of anti-government demonstrations in 2011 and has now killed more than 280,000 people and displaced millions.
Government troops, backed by Russian air strikes, in early June pushed into Raqa for the first time since 2014, aiming for the country`s largest dam at Tabqa on the Euphrates River.
"Daesh (IS) has managed to drive out regime troops from the administrative borders of Raqa province after a fierce counter-offensive," said the Syrian Observatory for Human Rights.
It said the jihadists had sent hundreds of reinforcements from their de facto capital of Raqa city to defend Tabqa, which also has an air base, located 50 kilometres to the west.
"More than 40 members of the pro-regime forces were killed," said the Britain-based Observatory, which relies on a vast network of sources on the ground for its information.
Jihadist losses were unavailable.An initial IS offensive on Sunday failed but a second attack seized many positions southwest of Tabqa.
On a separate front, IS also launched a surprise assault from another stronghold in Raqa province, killing residents of two villages it recaptured from US-backed fighters.
IS had dispatched a small group of jihadists -- including one driving an explosives-laden car -- into villages southeast of Manbij.
The villages had been seized in recent weeks by the US-backed Syrian Democratic Forces (SDF).
The US-led coalition backing the SDF carried out a barrage of air strikes Monday to defend the villages, said Syrian Observatory head Rami Abdel Rahman.
At least four SDF fighters were killed in the clashes and many more were wounded.
The SDF -- a Kurdish-Arab alliance with air support from the coalition -- encircled Manbij nearly 10 days ago.
But since then, they had been slowed by almost daily suicide bombings as IS puts up a fight for the town.
Held by the jihadists since 2014, Manbij was a key stop along IS`s supply route from the Turkish border southeast through the town of Tabqa and on to the city of Raqa.