BEIRUT: The Islamic State group launched a surprise assault Monday near its besieged stronghold in northern Syria, killing residents of two villages it recaptured from US-backed fighters, a monitor said.
The Syrian Observatory for Human Rights said IS had dispatched a small group of jihadists -- including one driving an explosives-laden car -- into villages southeast of their bastion of Manbij.
The villages had been seized in recent weeks by the US-backed Syrian Democratic Forces in their push for Manbij.
"IS is trying to defend Manbij by sending fighters from outside the town to attack the SDF in these villages," said Rami Abdel Rahman, the director of the Britain-based Observatory.
"Daesh executed residents," he added, using an Arabic acronym for IS.
The Manbij clashes killed five SDF fighters and coalition air strikes left 42 jihadists dead on Monday, said the Observatory, which relies on a vast network of sources on the ground for its information.
The US-led coalition backing the SDF carried out a barrage of air strikes on Monday to defend the villages, said 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 US-led coalition -- encircled Manbij nearly 10 days ago.
But since then, they have 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 its de facto Syrian capital of Raqa.
IS is also mounting a fierce defence of Tabqa, which has been under attack by Russian-backed regime forces since early June.
On Monday, the Observatory said, a failed IS counterattack against regime fighters southwest of Tabqa killed at least 14 jihadists and six government loyalists.
"IS dispatched 300 fighters from Raqa to Tabqa to help defend the town," Abdel Rahman said.
The jihadists launched a second counterattack in the afternoon, seizing many positions southwest of the town and pushing pro-regime fighters some 20 kilometres (12 miles) back from Tabqa airport.
IS has lost 21 jihadists since it began its assault on the region on Sunday, while 32 government loyalists have also been killed, according to the Observatory.
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.