Syrian fighter jets have conducted two devastating bombing runs on the rebel-held town of Azaz, flattening a string of houses and killing at least 30 people including children, activists said.
Hundreds of other residents fled across the border to Turkey after the attack on Wednesday, news agencies and witnesses said.
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"Bashar did this. God help us, these animals will kill us all," said one man, hoisting a bloodied arm that had been piled up on the pavement outside the hospital in Azaz after the bombardment.
A correspondent with AFP news agency said at least 10 houses had been flattened in the bombardment. Azaz, which lies just to the north of the main battleground city of Aleppo, is often used as a rear base by rebel Free Syrian Army (FSA) fighters.
The director of the UK-based Syrian Observatory for Human Rights, Rami Abdel Rahman, said more than 20 people were killed in the attack, the latest atrocity blamed on the regime of Syrian President Bashar al-Assad.
Witnesses said shortly after the attack that at least five bodies had been pulled from the rubble and many more were still trapped.
"This was a civilian area. All these houses were packed with women and children sleeping during the fast," said witness Abu Omar, a civil engineer in his 50s, referring to the dawn-to-dusk fast Muslims observe during Ramadan.
Witnesses and FSA forces who reinforced security around the town after the strike said the jet fired twice, targeting a makeshift media centre used by foreign reporters in the second, smaller strike.
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The attack came amid heavy shelling of several districts of Aleppo, regarded as a pivotal battleground in the conflict that is now entering its 18th month and has killed more than 23,000 people, according to activists.
The Observatory said the strike by a MiG fighter jet targeted a former Baath Party headquarters which had been taken over by rebel groups.
"The whole of the area was flattened," Abdel Rahman said. "Those killed included civilians and fighters, but what is clear is that there was a Free Syrian Army base there."
Dozens of people, many wailing and shouting, were climbing over the rubble, trying to pull out victims.
"That's it, I'm leaving for Turkey with my family today. Life here is impossible," said another witness who gave his name as Jomaa. "If you come to the basement of my house now, there are 15 women who are afraid to go out. This is what Bashar does to us."
At the local hospital, people brought in the body of a little girl apparently aged no more than four.
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Footage from an amateur video distributed by the Observatory also showed the immobile, dust-covered hand of a little girl, likely dead, reaching out from under ruined buildings.
Witnesses said the bomb must have weighed at least half a tonne and the impact shattered windows up to four blocks away. Residents insisted there was no rebel base where the bomb struck, but some said the families of FSA fighters lived there.
On the pavement outside the hospital, body parts had been heaped in a pile under a blanket.
"Nobody knows how high the toll will climb now. It could take days to finish searching through the rubble," said Abu al-Baraa, a doctor who had just arrived in Syria from Saudi Arabia to help.
"I'm a radiologist, not a surgeon, but I'll do anything I can to help," said Abu al-Baraa. There is only one other doctor at Azaz hospital, an anaesthetist.
Hundreds of people, most of them women and children, fled across the border to Turkey after the air strike, with families carrying bags of clothes and boxes of food on their heads, an AFP correspondent said.
"It was a massacre, an entire family like mine was exterminated," said one woman who refused to give her name.
In Aleppo itself, a new front had reportedly opened in the northeastern district of Baaideen, forcing residents to flee as regime forces pounded the area using tanks and warplanes.
Abu Ubayda, a local rebel commander, said regime forces were trying to encircle the FSA between Baaideen and southwestern district of Salaheddin which the government retook last week.