DAMASCUS, Syria -- Syrian rebels shelled a government-held neighbourhood in the northern city of Aleppo early Saturday, killing at least five people and wounding dozens, Syrian state television and an activist group reported.

Hours after the shelling, helicopter gunships struck a market in Aleppo's rebel-held neighbourhood of Maadi in apparent retaliation, according to the Britain-based Syrian Observatory for Human Rights and the opposition-run Aleppo Media Center.

The Observatory said the airstrike killed eight people and wounded dozens, some seriously. The Aleppo Media Center reported casualties but did not provide a precise toll.

The violence came as the head of the U.N. agency for Palestinian refugees planned to undertake an "urgent mission" to Damascus later Saturday amid concerns over the situation in the Palestinian refugee camp of Yarmouk, most of which has been captured by the Islamic State group.

State TV said the shelling on the predominantly Christian and Armenian neighbourhood of Suleimaniyeh in Aleppo early Saturday killed nine people, wounded another 50 and damaged several buildings.

The Observatory, which has a network of activists around the country, said the shelling killed five and wounded "tens."

Syrian rebels have shelled residential areas in government-held parts of the contested city in the past, killing hundreds of people. Government warplanes have dropped explosives-filled barrels on rebel-held neighbourhoods in Aleppo and other cities, killing thousands.

State TV said the rebels shelled the neighbourhood with a so-called Hell Cannon, a crude, locally made weapon that fires gas cylinders filled with explosives. The projectiles cause widespread damage and cannot be precisely targeted. The TV showed a building with its top three stories collapsed.

Aleppo, Syria's largest city and its former commercial capital, became a key front in the civil war after rebels launched an offensive there in July 2012.

Pierre Krahenbuhl, head of the U.N. agency for Palestinian refugees, known as UNRWA, was meanwhile set to meet with Syrian and U.N. officials in Damascus about the humanitarian situation in Yarmouk camp, agency spokesman Chris Gunness said.

Gunness said in a statement that there are deepening concerns over the safety of some 18,000 Palestinian and Syrian civilians, including 3,500 children, who remain in the camp.

Islamic State fighters overran much of Yarmouk last week, establishing a foothold in the Syrian capital for the first time. The incursion is the latest trial for Yarmouk's residents, who have already suffered through a devastating two-year government siege, starvation and disease.

Residents say there is barely enough food and water, and hospitals have long run out of drugs and supplies.

The Syrian government has said it will launch a military operation in Yarmouk to evict militants, which could cause even more devastation.