Areas of Syria's Hama province captured by rebel fighters came under heavy air attack on Thursday as pro-government forces sought to counter a major rebel assault in an area of strategic importance to President Bashar al-Assad.
The offensive that began on Tuesday is the biggest coordinated rebel assault in Hama province since 2014, according to the Syrian Observatory for Human Rights monitoring group.
The UK-based Observatory said at least 25 people, including six children, had been killed in overnight air strikes on Wednesday.
Syrian state television said on Thursday that the air force had carried out "concentrated strikes" against what it described as "terrorists" in the area, saying tens of them had been killed.
The rebel push in Hama marks a new challenge for Assad and his allies in a part of Syria where the embattled leader has tried to consolidate his grip on power against a five-year-long push to depose him.
An official in one of the rebel factions waging the attack, Jaish al-Nasr, told Reuters news agency that both Syrian and Russian jets were involved in what he described as heavy air strikes. Russia has been bombing anti-Assad forces for almost a year.
The rebel alliance, which includes elements of the Free Syrian Army (FSA) and Jund Al-Aqsa group, aims to take control of the airport in Hama, from which regime helicopters fly regular sorties against opposition fighters.
"They are about 10 kilometres from the airport" in Hama, Syria's fourth-largest city, said Observatory director Rami Abdel Rahman, whose group relies on a broad network of sources inside Syria.
The rebels are also probably seeking to ease pressure on opposition fighters in the battleground northern city of Aleppo by distracting regime forces.
In four days, the rebel alliance has seized control of 14 villages, mainly in the north of Hama province, including the towns of Halfaya and Suran.
They were also threatening to capture the historic Christian town of Mahrada, just west of the major north-south highway linking the capital, Damascus, with the northern city of Aleppo.
The targeted areas are populated by Christians and Alawites loyal to the government and are close to the mountain heartland of Assad's Alawite sect.
"We will target those who open fire on us," Jamil Saleh, a rebel commander in Hama, told Al Jazeera. "We won't target civilians at all... we are fighting for our land. Our enemy is the army."
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The Observatory said the air strikes that killed 25 people hit a road between the town of Latamenah and Idlib province, an area of northwestern Syria mostly under rebel control.
A Syrian military source told Reuters that the air force had destroyed dozens of rebel vehicles and the militants riding in them on a road from Latamenah to Idlib.
Hama province is of vital strategic importance to Assad, as it separates opposition forces in rebel-controlled Idlib from Damascus to the south and the government-controlled coast to the west.
In 2013, a major push by rebel groups to capture Hama was repelled by government forces after reinforcements were sent to the area.
Major demonstrations erupted in Hama in 2011 during the outbreak of Syria's civil conflict, but were quickly suppressed in a deadly government crackdown.
Assad's father and predecessor Hafez al-Assad brutally put down a Muslim Brotherhood uprising in Hama city in 1982, killing thousands of people.
Source: Al Jazeera News and agencies