Occasional skirmishes erupt there but the frontline has been quiet in recent months.
The bombing raids hit a string of towns and villages in the northwest province of Idlib, which is nearly entirely controlled by various jihadist and hardline rebels.
The so-called Syrian Observatory for Human Rights also confirmed the attack, saying, "The overnight attack on al-Foua and Kefraya was the fiercest in three years". "He calls for a full investigation into the attacks, especially allegations that there was also a second strike targeting first responders, to establish accountability", UN Secretary-General Spokesman Stephane Dujarric said in a statement.
Six Syrian pro-regime fighters and at least three from HTS were killed in the fighting, which continued on Sunday.
In eastern Syria, Syrian troops backed by Russian air forces repelled a militant attack on the historic town of Palmyra, according to state news agency SANA.
In apparent retaliation for the attack, Syrian warplanes pounded several towns around Fuaa and Kafraya on Sunday, AFP's correspondent and a monitor said.
Fuaa and Kafraya are the only two places in Syria now designated as besieged by the United Nations after the government recaptured the Yarmuk Palestinian camp in southern Damascus.
Panos Moumtzis, the United Nations regional humanitarian coordinator, called on major powers to broker a settlement to end the war and avoid a bloodbath in Idlib.
Troops have employed the tactic alongside heavy bombing to cut off food and medicine to rebel-held areas, then coerce people to agree to leave in population transfers.
More than 60 people were also injured in the attacks that took place in the village of Zardana.
Last year, the Syrian government and militants reached a deal, known as the "Four Towns Agreement", that allowed the evacuation of residents of the two towns.
On Friday, air raids believed to have been carried out by Russian Federation on a village in Idlib killed at least 44 people, inflicting the highest death toll in a single attack on the region this year.
They came under siege in 2015, when rebels ousted regime forces from a vast majority of Idlib province.
Currently, the Syrian government is fighting rebels in Idlib, the oppositions' last major stronghold in the country.
In retaliation, the IS cell executed five HTS fighters it had abducted in Idlib, the Britain-based Observatory said Sunday.
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