At least 27 civilians were killed on Sunday in a series of airstrikes by unidentified jets on the rebel-held northwestern Syrian city of Idlib and a nearby town, a monitoring group reported.

Some 21 people were killed and dozens injured in raids that hit a marketplace and other locations in Idlib, the Syrian Observatory for Human Rights said.

Two adults and four children were killed in air raids on the nearby town of Maaret al-Nuaman, which lies on the highway between Damascus and Aleppo, the group reported.

"There were regime and allied Russian warplanes flying in the area today. But we do not know yet which planes had carried out the strikes," the Observatory’s head Rami Abdel-Rahman told dpa.

Activists based in Idlib city put the number of dead higher, saying that at least 30 people were killed in the bombing of the market alone.

Footage posted online by activists showed one place shrouded in thick smoke amid destroyed buildings and damaged vehicles.

Sunday's bombardment was the latest in a spate of aerial attacks on the mostly rebel-held Idlib province. Last month, at least 30 people were killed in airstrikes on tent settlements sheltering refugees in a rebel-held area in the province.

Idlib is held by a coalition of mostly hardline Islamist rebel factions, including al-Qaeda’s Syria branch al-Nusra Front. 

Al-Nusra and the Islamic State extremist militia are excluded from a fragile truce that was brokered by the United States and Russia and went into effect in most Syria in February. 

Islamic State has carried out a series of counter-attacks in an attempt to break a US-backed siege on a strategic city held by the radical group in northern Syria, the Observatory said on Sunday.

The US-backed Democratic Forces of Syria (DFS) - an alliance of Syrian Kurds and Arabs - have besieged the Islamic State-controlled northern city of Minbij since Friday, cutting off a major supply route between the radical group's de facto capital in al-Raqqa in north-eastern Syria and the Turkish border.

The militants' counter-attacks, which started late Saturday, triggered clashes with the DFS, backed by US-led air raids that targeted Islamic State positions in and around Minbij, the Britain-based Observatory.

"The ongoing counter-attacks are desperate attempts by the organization to break the tight blockade," Abdel-Rahman said, referring to Islamic State.

"These attacks have made no effect so far."

He added that the al-Qaeda splinter group was getting reinforcements from al-Raqqa and the Islamic State-held town of al-Bab, located around 18 kilometres from Minbij.

On May 31, the DSF started an onslaught to seize Minbij, which has been under Islamic State control since 2014.

At least 223 Islamic State militants and 28 DFS fighters have since been killed in the Minbij-related violence, according to the Observatory that relies on a network of activists inside Syria.

Islamic State is locked in multi-front fighting in Syria, neighbouring Iraq and Libya in North Africa.

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