A Syrian man from Idlib is carried by Turkish medics wearing chemical protective suits to a hospital in the border town of Reyhanli in Hatay province, Turkey, April 4, 2017. [Photo/Agencies]

The US government believes the chemical agent sarin was used in the attack, a US government source said, adding it was "almost certainly" carried out by forces loyal to Syrian President Bashar al-Assad.

The Syrian military denied responsibility and said it would never use chemical weapons, echoing denials it has made over the course of the more than six-year Syrian civil war that has killed hundreds of thousands, created the world's worst refugee crisis and drawn in nations such as Russia, Iran and the United States.

The attack sparked political recriminations. US President Donald Trump blamed it on his predecessor Barack Obama's "weakness" on Syria, while a Syrian opposition figure said it was a consequence of recent US statements suggesting a focus on stopping Islamic State militants rather than ousting Assad.

If confirmed, the incident reported in the town of Khan Sheikhoun would be the deadliest chemical attack in Syria since sarin gas killed hundreds of civilians in Ghouta near Damascus in August 2013. Western states said the Syrian government was responsible for that attack. Damascus blamed rebels.

The head of the health authority in rebel-held Idlib province said more than 50 people had been killed and 300 wounded in the latest incident. The Union of Medical Care Organizations, a coalition of international aid agencies that funds hospitals in Syria, said the death toll was at least 100.

The British-based Syrian Observatory for Human Rights said the attack killed at least 58 people and was believed to have been carried out by Syrian government jets. It caused many people to choke and some to foam at the mouth.

Director Rami Abdulrahman told Reuters the assessment that Syrian government warplanes were to blame was based on several factors such as the type of aircraft, including Sukhoi 22 jets, that carried out the raid.

"We deny completely the use of any chemical or toxic material in Khan Sheikhoun town today and the army has not used nor will use in any place or time neither in past or in future," the Syrian army command said in a statement.

The Russian Defence Ministry, whose forces are backing Assad, said its aircraft had not carried out the attack. The UN Security Council was expected to meet on Wednesday to discuss the incident.

Reuters photographs showed people breathing through oxygen masks and wearing protection suits, while others carried the bodies of dead children. Corpses wrapped in blankets were lined up on the ground.

Activists in northern Syria circulated pictures on social media showing a man with foam around his mouth, and rescue workers hosing down almost-naked children squirming on the floor.