BEIRUT - At least 18 civilians were killed and 40 wounded in “probably Russian” air strikes on a rebel-held town in northwestern Syria on Sunday, a monitoring group said.

The strikes hit the Idlib province town of Ariha, which is controlled by the Army of Conquest, a rebel alliance of mainly Islamist groups, including Al-Qaeda affiliate Al-Nusra Front, the Syrian Observatory for Human Rights said.

Several areas of the town were hit and rescue operations are under way with several critically wounded, the Britain-based group said. The Army of Conquest alliance seized Ariha in May after heavy fighting with regime forces.

The Islamic State group has executed more than 3,500 people in Syria, including nearly 2,000 civilians, since declaring its “caliphate” in June last year, a monitor said Sunday.

In the last month alone IS executed 53 people - including 35 civilians - in areas it controls in Syria, the Syrian Observatory for Human Rights said. The new figures from the Britain-based Observatory bring to 3,591 the number of people executed by IS in Syria since it declared its Islamic “caliphate” after seizing control of large parts of the country.

Syrian President Bashar al-Assad on Sunday said his enemies have increased support for rebels fighting his regime as loyalists backed by Russia and Iran push an offensive to regain lost territory.

His remarks came as the Syrian Observatory for Human Rights said 18 civilians were killed and 40 wounded in “probable” Russian air strikes on a northwestern town held by a rebel alliance.

US senators John McCain and Lindsey Graham called Sunday for 100,000 foreign soldiers, most from Sunni regional states but also including Americans, to fight the Islamic State group in Syria.

Both McCain, the chair of the Senate Committee on Armed Services, and Graham, one of its members, sharply criticised current US strategy as insufficient and unsuccessful in defeating the militants.

That strategy has consisted of carrying out air strikes against IS in Iraq and Syria in support of local ground forces, which have also received weapons and training.

“I think 100,000 would be (the) total requirement,” McCain told journalists in Baghdad when asked about the size of the anti-IS force he and Graham were advocating for Syria.

A longtime ally of Damascus, Moscow escalated its support to President Bashar al-Assad on September 30 with an air campaign aimed at bolstering regime positions and backing ground operations by Syrian government troops.

Moscow says it targets the Islamic State group and other “terrorists”, but critics accuse it of targeting moderate and Islamist rebel fighters more than IS.

Syria’s neighbours have charged that Russian planes have strayed into their airspace on multiple occasions during the campaign. Russia carried out heavy raids in northern Syria this week after Turkey downed one of its jets in the area.

Turkey claims the warplane strayed into its airspace and ignored repeated warnings to change course, but Russia has insisted it did not cross the border. The incident has led to a sharp deterioration of relations between Ankara and its largest energy supplier.

The pilot of the Russian plane was shot dead by rebels as he parachuted down after ejecting, but his navigator was rescued by Russian and Syrian special forces.

The pilot’s body is to be handed over to a Russian representative after being retrieved from Syria, Turkish Prime Minister Ahmet Davutoglu said on Sunday.

A Russian warplane recently entered Israeli-controlled airspace from Syria but the intrusion was resolved without incident, Israeli Defence Minister Moshe Yaalon said on Sunday.

In early November, apparent Russian air strikes killed at least 11 civilians in two towns in Idlib province, the Observatory said.

More than 250,000 people have been killed since the Syrian conflict erupted in 2011.