Beirut: A suspected Russian airstrike has hit a displaced people's gathering in a town in Syria's last rebel enclave, killing at least 13, including a number of children, activists and a war monitor said.

The Britain-based Syrian Observatory for Human Rights said the strike, suspected to have been carried out by Russian aircraft on Friday, hit in the town of Hass south of Idlib province, where peopled displaced by the violence had congregated. The monitoring group said the attack killed at least four children.

Other activist-operated networks, including the Thiqa news agency, reported the attack and also put the death toll at 13.

Syrian troops have been on the offensive in Idlib and its surroundings, the last major rebel stronghold in Syria, since April 30 in a Russia-backed campaign. Fighting has picked up in recent days, after a collapsed short-lived cease-fire and government troops have been seizing territory on the enclave's edge.

Idlib and the surrounding rebel-held area is home to some 3 million people, many of them displaced in other battles around the war-torn country.

In recent weeks, more than 450,000 have been displaced within the area while more than 500 civilians were killed. The violence continued despite pleas from the United Nations and aid groups.

The fighting over the past days has been concentrated on two fronts as government forces march toward the town of Khan Sheikhoun.

Separately, Syrian state media reported that the country's air defences have responded to a "hostile target" and destroyed the missile before it reached a central Syrian town.

The SANA news agency said on Friday that the projectile had entered the Syrian airspace overnight from Lebanon's airspace, heading toward the town of Masyaf in Hama province.

The report suggested, without saying outright, that the missile was fired by Israel. SANA says the incoming projectile was destroyed before it reached its target.

There was no immediate comment from Israel.