Carrying bags of clothes and boxes of food on their heads, hundreds of Syrians are fleeing to Turkey after a massive Syrian air strike on the northern rebel bastion of Aazaz.

Wednesday's bombing flattened a string of houses and killed at least 46 people, with a Syrian watchdog reporting 31 killed and 200 more wounded and Turkey saying another 15 had died in its hospitals after crossing the border.

"That's it, I've had it. Nobody should have to live with this kind of fear in his heart. How could my children go to sleep if we didn't leave," said Abu Alaa, an Aazaz resident in his mid-40s.

Entire families could be seen filing past the immigration office at the crossing point into the Turkish town of Kilis.

"I've never seen anything like it," he said, shouting orders at his children and other relatives packed in the back of a pick-up truck.

Others walked from Aazaz with their belongings and were equally adamant they could no longer remain in their hometown, which lies just a few kilometres (miles) from the border with Turkey.

"It was a massacre, an entire family like mine was exterminated," said one woman who refused to give her name.

Turkey has been one of the main powers denouncing Syrian President Bashar al-Assad's regime and is being used as a rear base by several rebel groups.

Turkish authorities opened the border crossing to Syrians earlier this month.

The exodus was triggered after a Syrian fighter jet dropped a bomb on a heavily-populated area of Aazaz as many were sleeping during the Ramadan fast, levelling at least 10 houses, according to an AFP reporter at the scene.

Aazaz, just north of the main battleground city of Aleppo, was a rebel bastion used by Free Syrian Army fighters, but locals say many innocent people were killed in the air strike.

"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.

Human Rights Watch also said many of the victims were women and children.

"This horrific attack killed and wounded scores of civilians and destroyed a whole residential block," said Anna Neistat, HRW's acting emergencies director.

"Yet again, Syrian government forces attacked with callous disregard for civilian life," she said in a statement.

The regime launched the air strike shortly before a UN panel denounced Syrian forces and their militia allies for committing crimes against humanity, including murder and torture, during the conflict now in its 18th month.

The panel also accused the armed rebels who are battling the Assad regime of war crimes but to a lesser extent.

At the Syria-Turkey border, Samia, wearing a black head-to-toe veil, said she had come from Aleppo with her five children.

"My husband and eldest son stayed behind. One is looking after the shop and the other is fighting. But the rest of us had to leave. We know Assad will kill women and children," she said.