Syrian fighter jets screamed through the sky over a rebel-held town, dropping bombs that levelled the better part of a poor neighbourhood and wounded scores of people, many of them women and children buried under piles of rubble.
The British-based Syrian Observatory for Human Rights said 23 people died in the double air strike and more than 200 were wounded.
Mohammed Nour, a local activist reached by phone, put the death toll at 25. Neither figure could be independently confirmed.
Reporters from The Associated Press saw nine dead bodies in the immediate aftermath of the bombings in Azaz, including a baby.
The attacks sent panicked civilians fleeing for cover. So many were wounded that the local hospital locked its doors, directing people to drive to the nearby Turkish border so the injured could be treated on the other side. One person's remains were bundled into a small satchel.
A group of young men found a man buried in the wreckage of destroyed homes, his clothes torn and his limbs dirty, but still alive.
"God is great! God is great!" they chanted as they pulled him out and laid him on a blanket.
Nearby, a woman sat on a pile of bricks that once was her home, cradling a dead baby wrapped in a dirty cloth. Two other bodies lay next to her, covered in blankets. She screamed and threw stones at a TV crew that tried to film her.
The bombing of Azaz, 30 miles north of Aleppo, shattered the sense of control rebels have sought to project since they took the area from President Bashar Assad's army last month.
Azaz is also the town where rebels have been holding 11 Lebanese Shiites they captured in May.
The attack came on the same day the United Nations released a report accusing Assad's forces and pro-government militiamen of war crimes during a May bloodbath in the village of Houla that killed more than 100 civilians, nearly half of them children. It said rebels were also responsible for war crimes in at least three other killings.
The long-awaited report by the UN Human Rights Council marks the first time the world body has referred to events in Syria as war crimes - on both the government and rebel sides - and could be used in future prosecutions against Assad or others.
It said the scale of the Houla carnage indicated "involvement at the highest levels" of Syria's military and government. The council also said the conflict was moving in increasingly brutal directions on both sides.
The Organisation of Islamic Co-operation, comprising 57 member states, released a final statement yesterday from its two-day summit in Saudi Arabia's Muslim holy city of Mecca, urging support for the opposition.
The statement did not mention suspending Syria's membership, but OIC secretary general Ekmeleddin Ihsanoglu said after the summit that the organisation had agreed to do so. The move is largely symbolic.
Iran's foreign minister Ali Akbar Salehi, whose nation is Assad's most staunch regional supporter, said before the opening session in Saudi Arabia that suspending Syria would not resolve the issue of the unrest.
A wide-ranging tableau of violence and retributions yesterday reinforced the UN's warnings.
A blast in central Damascus rattled - but did not injure - UN observers, followed by the air strikes in Azaz. And in tense Lebanon, a powerful Shiite clan that backs Assad said it kidnapped at least 20 Syrians in retaliation for rebels holding one of their relatives captive in Syria.
The rebels accuse the Lebanese man of belonging to Hezbollah, a Shiite Lebanese group allied with Syria and Iran.
The bombing of Azaz brought into stark relief the limits of the rebels' expanding control of Syria's north.
In recent months, rebels have pushed the Syrian army from a number of towns in a swathe of territory south of the Turkish border and north of Aleppo, Syria's largest city. About a dozen destroyed tanks and army vehicles are scattered around Azaz, left over from those battles.
As the Assad regime's grip on the ground slips, however, it is increasingly targeting rebel areas with attack helicopters and fighter jets - weapons the rebels cannot challenge.
The first blast seemed to come out of nowhere, shaking the city's centre and sending up a huge grey cloud of smoke that sent terrified residents rushing through the streets looking for cover.
Not long after, another jet appeared, dropping bombs nearby.
"We were in the house and heard this plane overhead," said a 36-year-old woman who gave her name as Um Hisham. "There was this huge boom that made my mother pass out in the kitchen."
The revolt that started in March 2011 with protests calling for political change has killed more than 20,000 people, activists say.
The area appeared to be no more than a poor, residential neighbourhood with a few metal workshops, and residents said there were no rebel bases there, though they often do not speak openly about where rebels operate.
Azaz has considered itself "liberated" since rebel forces pushed out the army last month. Its largest rebel group, the Northern Storm Brigade, runs a prison and the nearby border crossing with Turkey.
Its political and media offices are less than a mile away from the bombing site, in the former local headquarters of Assad's Baath party.