KHAN SHEIKHUN, Syria (AFP) — A suspected chemical attack in rebel-held northwestern Syria killed dozens of civilians including children and left many more sick and gasping on Tuesday, causing widespread outrage.

The attack on the town of Khan Sheikhun killed at least 58 civilians and saw dozens suffering respiratory problems and symptoms including vomiting, fainting and foaming at the mouth, the Syrian Observatory for Human Rights monitoring group said.

At least 11 children were among the dead, the Observatory said, and an AFP correspondent in Khan Sheikhun saw many attached to respirators as they were treated for breathing problems.

Hours after the initial attack, air strikes also hit a hospital in the town where doctors were treating victims, the AFP correspondent said, bringing down rubble on top of medics as they worked.

Syria’s opposition blamed President Bashar al-Assad’s forces, saying the attack cast doubt on the future of peace talks.

A senior Syrian security source denied claims of regime involvement as a “false accusation”, telling AFP that opposition forces were trying to “achieve in the media what they could not achieve on the ground.”

If confirmed, it would be one of the worst chemical attacks since the start of Syria’s civil war six years ago.

The incident brought swift international condemnation, with France and Britain demanding an emergency UN Security Council meeting and President Francois Hollande denouncing a “massacre”.

British Foreign Secretary Boris Johnson called reports of the attack “horrific” and said the incident “must be investigated and perpetrators held to account”.

UN Syria envoy Staffan de Mistura said the attack was believed to be chemical and launched from the air, adding that there should be a “clear identification of responsibilities and accountability”.

The Observatory said the attack on a residential part of Khan Sheikhun came early on Tuesday morning, when a warplane carried out strikes that released “toxic gas”.

As well as those killed, at least 160 people were injured, it said, and many were dying even after arriving at medical facilities.

The monitor could not confirm the nature of the gas, and said the attacks were likely carried out by government warplanes.

“We heard strikes this morning… We ran inside the houses and saw whole families just dead in their beds. Children, women, old people dead in the streets,” resident Abu Mustafa said.

Russia’s military, which has been fighting in support of Assad’s government since September 2015, denied carrying out any strikes near the town.

The AFP journalist in Khan Sheikhun saw a young girl, a woman and two elderly people dead at a hospital, with foam still visible around their mouths.

Doctors at the facility were using basic equipment, some not even wearing lab coats, and attempting to revive patients who were not breathing.

A father carried his dead little girl, her lips blueish and her dark curls visible, wrapped in a sheet.

As doctors worked, a warplane circled overhead, striking first near the facility and then hitting it twice, inflicting severe damage and prompting nearly a dozen medical staff to flee.

It was not immediately clear how many people may have been injured or killed in the hospital strikes.

Speaking to AFP, medic Hazem Shehwan said victims were suffering from symptoms including “pinpoint pupils, convulsions, foaming at the mouth and rapid pulses.”

Khan Sheikhun is in Syria’s Idlib province, which is largely controlled by an alliance of rebels including former Al-Qaeda affiliate Fateh al-Sham Front.

The province is regularly targeted in strikes by the regime, as well as Russian warplanes, and has also been hit by the US-led coalition fighting the Islamic State group, usually targeting jihadists.

Syria’s leading opposition group, the National Coalition, blamed Assad’s government for the attack and demanded the UN “open an immediate investigation” and hold those responsible to account.

“Failure to do so will be understood as a message of blessing to the regime for its actions,” it said.

Syria’s government officially joined the Chemical Weapons Convention and turned over its chemical arsenal in 2013, as part of a deal to avert US military action.