BEIRUT (AP) — A suspected chemical attack in a town in Syria's northern Idlib province killed dozens of people on Tuesday, Syrian opposition activists said, describing the attack as among the worst in the country's six-year civil war.

The Britain-based Syrian Observatory for Human Rights monitoring group put the death toll at 35, among them 10 children, while the Idlib Media Center said dozens had been killed.

The media center published video 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.

The activists did not release any details 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.

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.

There was no comment from the government in Damascus or any international agency on the attack.

Tuesday's reports came on the eve of a major international meeting in Brussels on the future of Syria and the region hosted by the EU's High Representative Federica Mogherini.

The Syrian Coalition, an opposition group based outside the country, said government planes carried out the airstrike on Khan Sheikhoun, south of the city of Idlib, the provincial capital.

It said the planes fired missiles carrying poisonous gases, killing dozens of people, many of them women and children. The coalition described the attack as a "horrifying massacre."

Photos and video emerging from Khan Sheikhoun show limp bodies of children and adults. Some are seen struggling to breathe; others appear foaming at the mouth.

A medical doctor going by the name of Dr. Shajul Islam for fears for his own safety said his hospital in Idlib province received three victims, all with narrow, pinpoint pupils that did not respond to light. He published video of the patients on his Twitter account.

Pinpoint pupils, breathing difficulties, and foaming at the mouth are symptoms commonly associated with toxic gas exposure.

The opposition's Civil Defense search-and-rescue group, which released photos showing paramedics washing down victims, has not published a casualty toll.

The activist-run Assi Press published video of paramedics carrying victims from the scene by a pickup truck. The victims were stripped down to their underwear. Many appeared unresponsive.