BEIRUT: Forty people were killed on Friday when rockets fired by Syrian government forces crashed into a market in a rebel-held area outside Damascus, a monitor said.

"There were 40 people killed and at least 100 wounded in the centre of Douma," a town on the eastern edges of the Syrian capital, Rami Abdel Rahman, head of the Syrian Observatory for Human Rights (SOHR), told AFP.

"There is still heavy fire now, with both rockets and mortars," he said, adding that the toll was expected to rise as people were still being pulled out of damaged buildings.

Rebel-held Douma lies in Eastern Ghouta, the largest opposition stronghold in Damascus province.

Government air strikes on Thursday hit a market and a hospital, killing at least nine people, the Observatory said.

An AFP photographer at the scene said Thursday's attack had wounded hospital staff, limiting the treatment available for the wounded on Friday.

He said the attack on Friday took place as residents gathered at the market, leaving corpses piled on top of each other.

According to Abdel Rahman, "Douma is one of the areas in Syria where there are the highest number of deaths since the beginning of the war."

Government forces regularly target the region with rocket fire, shelling, and aerial raids, and opposition groups there also fire rockets into the capital.

The Douma Coordination Committee, a local activist group, published a gruesome video on Friday of what it said was the aftermath of more than a dozen rockets landing on the market.

Blood-soaked bodies lay crumpled underneath tables of food and other goods, as men gathered around wounded people.

A young boy in a sky-blue sweater stood on the sidelines, looking stunned.

In August, 117 people were killed in a single day of air strikes in the town, causing a global outcry.

Throughout Syria's brutal war, both the government and opposition forces have been criticised by rights groups for indiscriminate fire on civilian areas.

More than 250,000 people have been killed since the conflict began in March 2011.