Beirut (AFP) - A Syrian brigadier-general was killed in a blast that targeted his car in downtown Damascus on Sunday, the Syrian Observatory for Human Rights monitor said.

The Britain-based group said "a brigadier general and approximately six people with him were killed in an explosion in the Adawi neighbourhood in the centre of the capital."

The powerful Ahrar al-Sham rebel group posted a video online claiming to show the attack on Brigadier-General Bassem Ali Muhanna's car with an improvised explosive device, in the early hours of Sunday.

A security source said Muhanna was a member of the Republican Guards, an elite unit of the army.

Elsewhere, jihadists shot down a government helicopter in northern Syria's Aleppo province, according to both the Observatory and supporters of the Islamic State group.

But state media said the helicopter had suffered technical problems.

The Observatory, which relies on a network of sources on the ground, said IS had brought down the helicopter after midnight near the Kweyris air base in the east of Aleppo province.

Its director, Rami Abdel Rahman, said at least one crew member had been killed but "the fate of the rest is unknown".

Jihadist accounts on Twitter said IS had downed the helicopter using anti-aircraft missiles.

They posted the names of three crew members they said had been killed and pictures of a helicopter in flames.

State television said, however, that "a helicopter crashed after takeoff from Kweyris airport in Aleppo province because of technical faults and the crew were killed".

IS fighters have surrounded the air base since March 2014 and have fought fierce clashes with its garrison.

- Palmyra 'massacre' unconfirmed -

The government has used helicopters to drop so-called barrel bombs on rebel-held areas of Aleppo province.

The crude weapons -- made from oil drums, gas cylinders or water tanks, packed with explosives and scrap metal -- have killed hundreds of civilians, drawing condemnation from human rights groups.

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President Bashar al-Assad has denied they are being used.

Elsewhere, the Observatory said the toll in regime air raids on Saturday in the eastern city of Deir Ezzor rose to 16, including six children from one family.

IS jihadists control most of Deir Ezzor province, including more than half of the provincial capital, which sits strategically on several key highways.

The Observatory also gave new details about the battle for a building in Jisr al-Shughur where regime forces were besieged by rebels until Friday.

The siege began when the town fell to rebels including Al-Qaeda affiliate Al-Nusra Front on April 25, but a group of regime forces got stuck inside the building.

Dozens of those trapped were able to flee on Friday as the opposition forces finally stormed the building.

But the Observatory said at least 75 regime forces were killed in the fight for the building and the subsequent evacuation.

It said another 73 soldiers had been taken hostage and 91 regime forces and their families had escaped and reached government lines.

Syrian media said government air strikes which provided the cover for the escape had killed 300 Al-Nusra fighters.

Also Sunday, state television reported what it called a "massacre" in the city of Palmyra, which was captured by IS on Thursday.

State media said 400 people had been killed, "mostly women, children and elderly people."

But the Observatory said there was no evidence of such mass killings.

"Several dozen people accused of ties to the regime have been executed, but the number does not exceed 35 since the town was captured," Abdel Rahman said.

One woman and two children were among those killed but the rest were men, added the Observatory director.