Air strikes believed to have been carried out by Russia killed at least 44 people in a village in Syria's rebel-held Idlib province on Thursday, the Syrian Observatory for Human Rights said. "War planes, which are likely Russian, targeted the village of Zardana in northern rural Idlib overnight and caused the highest death toll in a single attack on the region including 11 women and six children," Rami Abdel Rahman, the director of the Britain-based war monitor, said on Friday. The death toll is expected to increase as some of the 60 injured in the strikes were in a critical condition, he said. Rescue workers were still searching under the rubble for survivors. Syrian civil defence volunteers, also known as White Helmets, said the air strike killed 35 people and wounded 80. They told Associated Press that the dead included three of their members, who were killed as they evacuated the dead and wounded. Russia denied responsibility for the attack, saying it had not carried out any air strikes in Idlib province on June 7. The north-western Idlib region remains the largest populated area of Syria in the hands of insurgents fighting the Damascus government. In recent years, tens of thousands of fighters and civilians have fled there from parts of the country which the army has recaptured with the help of Russia and Iran. The air strikes came on the same day ISIS fighters killed 22 pro-regime fighters, including 9 soldiers and two officers, in surprise attacks in southern Syria. Twelve militants were killed in the assaults in the desert of the southern province of Sweida, the Observatory said. These were the first attacks of their kind in the area, where no ISIS presence had been noted in more than a year, Mr Abdel Rahman said. The 22 pro-regime combatants included nine Iranians and fighters belonging to pro-Iran Shiite militias, as well as two unidentified fighters, he said. ISIS has ramped up its attacks against pro-regime forces since its fighters left their last stronghold near Damascus in May, under an evacuation deal with the regime. The new toll brought to 184 the number of pro-regime troops killed in more than two weeks of extremist attacks across the country since May 22. At least 92 extremist fighters were killed during that same period. ISIS in 2014 proclaimed a cross-border "caliphate" in Syria and neighbouring Iraq, but has since lost most of that territory to separate offensives by Russia-backed regime troops and a US-backed Kurdish-Arab alliance. The extremists now control no more than three per cent of Syria, the Observatory says, mostly in the eastern desert near the border with Iraq. Earlier this week, ISIS attacks in the eastern province of Deir Ezzor killed 45 pro-regime fighters.