Suspected chemical attacks killed at least 100 people in Syria's rebel-held northwestern province of Idlib on Tuesday and left another 400 suffering from respiratory problems, a Syria medical relief group said.

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The death toll is likely to rise, according to the Union of Medical Care Organizations, a coalition of international aid agencies that funds hospitals in Syria and which is partly based in Paris.

The group said the village of Khan Sheikhoun to the south of Idlib had initially been hit before strikes on the White Helmets emergency services center in Khan Sheikhoun and the Al-Rahme hospital.

Open gallery view A man carries a wounded child following a suspected chemical attack, northern Idlib province, Syria, April 4, 2017. Credit: Uncredited/AP

"We have seen more than 40 strikes since 6:30 A.M," it said. "The toll continues to increase as do the strikes in the Idlib region as well as non-chemical attacks in Hama," the group said.

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Russia denied its warplanes were behind Tuesday's airstrikes, the official RIA news agency reported, citing the defense ministry.

Media outlets affiliated with the Assad regime quoted a senior Syrian Army official who refuted reports regarding the use of sarin gas or any other chemical weapon.

"We haven’t used such a weapon, not now and not in the past, and we won't use it in the future," he was quoted as saying.

The attack caused many people to choke or faint, and some had foam coming out of their mouths, the Syrian Observatory for Human Rights said, citing medical sources who described it as a sign of a gas attack.

The airstrikes also wounded more than 60 people in the town of Khan Sheikhoun in rebel-held Idlib, the Observatory said.

France called for a UN Security Council meeting in the wake of the attack, AFP reported.

Open gallery view A Syrian man receives treatment following a suspected toxic gas attack in Khan Sheikhun, a rebel-held town in the northwestern Syrian Idlib province, on April 4, 2017. Credit: MOHAMED AL-BAKOUR/AFP

EU foreign policy chief Federica Mogherini said President Assad's regime bears responsibility for the "awful" chemical attack.

Activists in northern Syria circulated pictures on social media showing a reported victim with foam around his mouth, and rescue workers hosing down almost naked children squirming on the floor.

A Syrian military source said last week allegations that government forces were using chemical weapons were "devoid of truth."

On Monday, Syrian jets bombed residential areas in the eastern countryside of Damascus, killing and injuring dozens in some of the heaviest bombing raids on the main rebel enclave near the capital in months, residents and activists said.

At least 22 people were killed and scores injured after four aerial raids hit a crowded district in the city of Douma, the main urban center of the Eastern Ghouta rebel stronghold to the east of the capital.