A military air strike on the town of Salqeen in Syria's mostly rebel-held province of Idlib on Monday killed 21 people, including eight children, the Syrian Observatory for Human Rights reported.
The Britain-based watchdog gave an initial toll of 36 people -- 29 civilians, five rebels and two soldiers -- killed nationwide on Monday.
"Twenty-one civilians, including eight children, were killed in bombing on the town of Salqeen and the number is expected to rise because many are in critical condition," the Observatory said, citing an activist and a medical source.
"This was a single air strike that hit several buildings. We expect the number of dead to increase because 30 people were living in this area, some of whom are critically wounded and others are buried under the rubble," Observatory director Rami Abdel Rahman told AFP.
In a video released by activists from Salqeen, which could not immediately be verified, a number of the victims are seen piled in the back of a pick-up truck, their bodies charred black with limbs torn off.
"My God, my son is dead," a man wails as he looks on at the bloody disfigured corpses, finally putting his hand over his eyes.
According to Abdel Rahman, among the dead are three children from the same family.
In another video, the bodies of three small children, probably the same ones, are shown lying on a bedsheet with their faces bloody and mutilated.
"These children are from one family," a man tells the person filming.
Other footage shows residents and two Kalashnikov-toting men wearing fatigues walking over mounds of rubble in Salqeen. The camera pans to nearby buildings which have had their entire sides blown off, as a crying child can be heard in the background.
The videos cannot be independently verified due to severe restrictions on foreign media imposed by the regime.
Salqeen lies about five kilometres (three miles) from the Turkish border.
"Turkish ambulances are waiting at the Bab al-Hawa crossing for the wounded, as there are no decent hospitals on the Syrian side," Abdel Rahman said.
He said that regime forces had tried to enter the town in the morning but they were pushed back by rebel fighters. The air strike followed soon afterwards.
Elsewhere in the province, three rebels were killed on Monday in clashes with troops in the town of Kfar Takhareem.
The Observatory also reported that five people, including a woman and her father, were killed when loyalist forces Monday shelled the town of Tafas, in the southern province of Daraa where the revolt against President Bashar al-Assad's autocratic rule erupted in March last year.
In other violence in Daraa on Monday, a rebel was killed in clashes while two army officers died in an attack on their vehicle, all in the town of Naayma, which came under heavy shelling after the military withdrew its tanks.
Elsewhere, a man was killed in army shelling of rebel zones in the central province of Homs, while a rebel commander died in fighting with regime forces near the capital Damascus, the watchdog said.
And in Hama city, a man was shot dead by troops while one woman was killed in shelling in the same province.
The uprising, which began with peaceful protests for reform which were brutally crushed, has since turned into a civil war pitting mainly Sunni rebels against Assad's minority Alawite-dominated regime. Activists say more than 30,000 people have died in the conflict.
The Observatory, which relies on its information from a network of activists and medics on the ground, said that on Sunday alone, 126 people were killed across the country -- 48 civilians, 63 soldiers and 15 rebels.