The Syrian Observatory for Human Rights (SOHR) reports at least 58 people have been killed and dozens wounded in a suspected chemical attack on a rebel-held town in north-western Syria.

It says that strikes on Khan Sheikhoun by Syrian government or Russian jets had caused many people to choke.  Later, aircraft reportedly fired rockets at local clinics treating survivors.

Syrian Army has rejected claims of chemical weapons use in Idlib and blamed militants. 

Russia's defense ministry meanwhile insisted it had not carried out any air strikes in the vicinity.

If confirmed, it would be one of the deadliest chemical attacks in Syria's civil war.

The warplanes are reported to have attacked Khan Sheikhoun, about 50 kilometers south of the city of Idlib, early on Tuesday, when many people were asleep.

Hussein Kayal, a photographer for the pro-opposition Edlib Media Center (EMC), told the Associated Press that he was awoken by the sound of an explosion at about 06:30 (03:30 GMT).  When he reached the scene, there was no smell, he said. He found people lying on the floor, unable to move and with constricted pupils, he added.

Mohammed Rasoul, the head of a charity ambulance service in Idlib, told the BBC that his medics had found people, many of them children, choking in the street.

The SOHR quoted medics as saying that they had been treating people with symptoms including fainting, vomiting and foaming at the mouth.

An AFP news agency journalist saw a young girl, a woman and two elderly people dead at a hospital, all with foam still visible around their mouths.

The journalist also reported that the same facility was hit by a rocket on Tuesday afternoon, bringing down rubble on top of doctors treating the injured.

The source of the projectile was not clear, but the EMC and the opposition Local Co-ordination Committees (LCC) network said warplanes had targeted several clinics.

According to the BBC, pro-government journalists later cited military sources as saying there had been an explosion at an al-Qaeda chemical weapons factory in Khan Sheikhoun that was caused either by an air strike or an accident.

The SOHR put the death toll at 58, including 11 children, but Mr. Rasoul reported that 67 people had been killed and that 300 were injured.

The pro-opposition Step news agency meanwhile said 100 had died.

One aid agency, the Union of Medical Care and Relief Organizations (UOSSM), put the death toll at more than 100 and said that the figure was likely to rise.