The UK-based Syrian Observatory for Human Rights said at least 25 civilians had been killed in strikes on a road near the town of Latamina overnight.

It also reported rebel gains to the south, near Maardis, 11km (seven miles) from the provincial city of Hama.

But state media said army operations in the area had left 50 "terrorists" dead.

Troops and allied militiamen had been involved in heavy clashes near Suran and al-Buwaida - both just to the north of Maardis - and destroyed a number of tanks, a military source told the official Sana news agency.

"Concentrated strikes" had also targeted "terrorist gatherings" outside nearby Taibat al-Imam, killing several people, the source added.

The Syrian Observatory said the rebel offensive, which began on Tuesday, was the biggest co-ordinated operation in Hama province since 2014.

It involves the jihadist group Jund al-Aqsa and factions fighting under the banner of the Western-backed Free Syrian Army, according to one of the FSA groups.

Sana's report did not mention any territorial losses, but if the rebels were to take control of Hama or the north-south motorway it sits on, they would sever a major government supply line.

Local activist Ahmed al-Ahmed told the Associated Press that the government's forces around Hama might have been weakened by its decision to send reinforcements to the divided northern city of Aleppo, where fighting has escalated.

The humanitarian adviser to the UN special envoy for Syria said on Thursday that it was urgent a 48-hour "humanitarian pause" in Aleppo be agreed so that supplies of food and medicine could be delivered safely to the city's rebel-held east and government-controlled west.

"We were informed today that there are now 4,000 food rations left, that would be enough for 20,000 people and the population of east Aleppo is a quarter of a million," Jan Egeland said.

The UN envoy, Staffan de Mistura, said discussions on the humanitarian pause, as well as a wider cessation of hostilities, were currently taking place in Geneva between "very senior military, security and diplomatic officials" from Russia and the US, which support Syria's government and opposition respectively.

Mr de Mistura also expressed concern at the government's apparent "strategy" of besieging and bombarding rebel-held areas until their civilian populations surrender, a week after the 6,000 remaining residents of the Damascus suburb of Darayya struck a deal to be evacuated.

There were "indications that after Darayya we may have other Darayyas", Mr de Mistura warned, pointing to the government's sieges of al-Wair, a suburb of the central city of Homs, and Muadhamiya, another suburb of Damascus.