The death toll from a suspected chemical weapons attack on a rebel-held Syrian town has risen to 72, 20 of them children, a monitoring group said today.

"There were also 17 women among the dead and the death toll could rise further because there are people missing," the Syrian Observatory for Human Rights said.

The attack in the town of Khan Sheikhun left dozens suffering respiratory problems and symptoms including vomiting, fainting and foaming at the mouth, the Britain-based Syrian Observatory for Human Rights said. A hospital in the town where doctors were treating victims of the attack was also bombarded, an AFP correspondent said.

The entrance of the building was hit, bringing down rubble on top of medics who had earlier been seen dousing a steady stream of arrivals to wash away chemical residue. The violence came as the European Union and UN hosted a conference in Brussels on Syria's future, with confusion over Washington's position on the issue of Syrian President Bashar al-Assad's future.

The Observatory said the attack on a residential part of Khan Sheikhun came in the early hours of Tuesday, when a warplane carried out strikes that released "toxic gas". It said 11 children were among the dead, with at least 160 injured, and that many people were dying even after arriving at medical facilities.

The monitor could not confirm the nature of the gas or whether the strikes were carried out by Syrian warplanes or those of government ally Russia. It relies on a network of sources inside Syria and says it determines whose planes carry out raids according to type, location, flight patterns and munitions used.

An AFP journalist in Khan Sheikun saw a young girl, a woman and two elderly people dead at the hospital prior to the bombardment, all with foam still visible around their mouths. Doctors at the facility had been using basic equipment, some not even wearing lab coats, and attempting to revive patients who were not breathing.