BEIRUT, April 4 (Reuters) - A suspected Syrian government chemical attack killed scores of people, including children, in the northwestern province of Idlib on Tuesday, a monitoring group, medics and rescue workers in the rebel-held area said.

The U.S. government believes the chemical agent sarin was used in the attack, a U.S. 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.

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 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 at least 100 people had died.

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.

Damascus has repeatedly denied using such weapons during the six-year civil war, which has killed hundreds of thousands and created the worlds worst refugee crisis.

The Russian Defence Ministry said its aircraft had not carried out the attack. The U.N. 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.

A senior U.S. State Department official said it appeared that the attack blamed on Assad amounted to a war crime.