A suspected chemical attack in a rebel-held Syrian town killed 100 people and injured 400 others, a medical relief group said, and some medics treating the wounded were later struck by rubble when an aircraft reportedly bombed a hospital.
A hospital in Syria's northern Idlib province was hit soon after the area was bombarded with a suspected chemical agent, an AFP correspondent reported.
The Britain-based Syrian Observatory for Human Rights monitoring group said there were 11 children among the dead.
The Syrian medical relief group UOSSM reported that the overall death toll had been elevated to 100, according to Reuters.
Mohammed Rasoul, the head of a Syrian ambulance service, told the BBC that first responders found people choking in the streets.
"Our team is still there, moving patients from one place to another because of overcrowded hospitals," he said.
"I am speaking to my team and they are doing fine, but the situation over there is very bad and most of those who are suffering are children."
The media center published footage of medical workers appearing to intubate an unresponsive man stripped down to his underwear and hooking up a little girl foaming at the mouth to a ventilator. There was no comment from the government in Damascus or any international agency in the immediate aftermath of the attack.
It was the third claim of a chemical attack in just over a week in Syria.
The previous two were reported in Hama province, in an area not far from Khan Sheikhoun, the site of Tuesday's alleged attack.
Tuesday's reports came on the eve of a major international meeting in Brussels on the future of Syria and the region, to be hosted by the EU's High Representative Federica Mogherini.
The Syrian American Medical Society, which supports hospitals in opposition-held territory, said it had sent a team of inspectors to Khan Sheikhoun before noon and an investigation was underway. The Syrian activists had no information on what agent could have been used in the assault.
They claimed the attack was caused by an airstrike carried out either by the Syrian government or Russian warplanes.
It was also not immediately clear if all those killed died from suffocation or wounds sustained in the airstrikes.
Makeshift hospitals soon crowded with people suffocating, activist said.
Mohammed Hassoun, a media activist in nearby Sarmin -- also in Idlib province where some of the critical cases were transferred -- said the hospital there is equipped to deal with such chemical attacks because the town was also struck, early on in the Syrian uprising.
The Sarmin hospital is about 31 miles away from the scene of the attack.
"Because of the number of wounded, they have been distributed around in rural Idlib," he told The Associated Press by phone.
"There are 18 critical cases here.
They were unconscious, they had seizures and when oxygen was administered, they bled from the nose and mouth."
Hassoun, who is documenting the attack for the medical society, said the doctors there have said it is likely more than one gas.
"Chlorine gas doesn't cause such convulsions," he said, adding that doctors suspect sarin was used. Hussein Kayal, a photographer for the Idlib Media Center, said he was awoken by the sound of a bomb blast around 6:30 a.m.
When he arrived at the scene there was no smell, he said. He found entire families inside their homes, lying on the floor, eyes wide open and unable to move.
Their pupils were constricted. He put on a mask, he said.
Kayal said he and other witnesses took victims to an emergency room, and removed their clothes and washed them in water.
He said he felt a burning sensation in his fingers and was treated for that.
A Turkey-based Syrian man whose niece, her husband and one-year-old daughter were among those killed, said the warplanes struck early, as residents were still in their beds.
He spoke on condition of anonymity because he feared for the safety of family members back in Syria.
The province of Idlib is almost entirely controlled by the Syrian opposition.
It is home to some 900,000 displaced Syrians, according to the United Nations.
Rebels and opposition officials have expressed concerns that the government is planning to mount a concentrated attack on the crowded province.
Claims of chemical weapons attacks, particularly the use of the chlorine agent, are not uncommon in Syria's conflict.
The worst attack was what a U.N. report said was an attack by toxic sarin gas in August 2013 on the Damascus suburb of Ghouta that killed hundreds of civilians.