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".
"We have seen in the last few weeks an escalation of even fighting between these groups that have taken place", he said, noting that fighting had been occurring around health facilities and other civilian areas.
Idlib, which lies on the border with Turkey, has seen its population balloon to around 2 million people in recent years as fighters and civilians evacuated from other opposition areas are dumped there.
"Hayat Tahrir al-Sham (HTS) began heavily shelling the two villages, and broke into Fuaa to attack local fighters", said Observatory chief Rami Abdel Rahman.
Fuaa and Kafraya are the only two places in Syria now designated by the United Nations as besieged after the government recaptured the Yarmouk Palestinian camp in southern Damascus.
The first four months of the year have seen more than 900,000 people flee their homes inside Syria.
Syria's Assad dismisses reports Russia cooperating with Israel on air strikes
At the same time, when asked about his time in London as a medical student, he admitted there were things he missed about the city.
Rebels overran most of Idlib in 2015, but the mainstream opposition's influence has dwindled as jihadists like HTS have solidified their grip on the province.
On Friday, Russian Defense Ministry rebutted allegations of an airstrike carried out by a Russian warplane on June 7 near the village of Zardana in Syria's Idlib province.
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.
"He recalls that Idlib is part of the de-escalation agreement reached in Astana (capital of Kazakhstan) and calls on its guarantors to uphold their commitments", Dujarric said.
Syrian forces later moved to liberate other militant-held areas in the provinces of Homs and Hama.
HTS and its allies control around 60 per cent of the province, its local rivals hold about a third, and the regime controls about 10 per cent, according to the observatory. Russian Federation entered the Syria conflict in September 2015, offering support to Damascus, which changed the course of the war, allowing government troops to retake more than half the country from opposition and Daesh terrorists.
The White Helmets say at least 16 more people were killed by Syrian government airstrikes over the weekend.