Syrian government helicopters dropped barrel bombs on opposition-held districts of the northern city of Aleppo today, killing at least 31 people including several children, an NGO has said.

Some of the wounded were in a serious condition after the strikes on the Sukkari and Ashrafiyeh neighbourhoods, the Syrian Observatory for Human Rights said.

Two bombs hit Sukkari minutes apart, with the second catching helpers who had gone to the assistance of the casualties from the first.

"We were sleeping safely when the first barrel dropped around midnight," said resident Abu Mohammad.

"And when people came to the rescue, a second barrel dropped, so all those people who were on the site were dead.

"We don't have military bases in our area. All people here are civilians. Why is he (President Bashar al-Assad) attacking us?" he asked.

Syria's regime has waged a fierce aerial offensive against rebel-held areas of Aleppo since December.

The Observatory said in May that the campaign had killed nearly 2,000 people, more than a quarter of them children, since the beginning of 2014.

Rights groups have decried the regime's use of barrel bombs as unlawful because they lack any guidance mechanism, causing indiscriminate casualties.

Tens of thousands of people have fled rebel-held districts of Aleppo in the face of the bombing.

Elsewhere, eight children were among 13 people killed by rebel shelling of a government-held area of Idlib province last night, the Observatory said.

An activist in Idlib said that among the children killed were two five-year-old boys, two aged seven and one aged eight.

More than 162,000 people have been killed in Syria since the uprising against Mr Assad's rule erupted in March 2011.