BEIRUT (AFP) -- The UN envoy to Syria condemned regime bombing of civilian areas as "totally unacceptable" after more than 90 people were killed in a day of heavy air raids.

Across the border in Iraq, pro-government forces pressed their operation aimed at sealing off jihadists who captured the city of Ramadi two weeks ago.

In Syria, barrel bombs dropped by President Bashar al-Assad's helicopters killed 71 civilians, including children, in the northern province of Aleppo on Saturday, a monitoring group said.

"The news of aerial bombing by Syrian helicopters on a civilian area of the Aleppo neighbourhood of Al-Shaar deserves the most strong international condemnation," UN envoy Staffan de Mistura said in a statement.

"The use of barrel bombs must stop," he said. "All evidence shows that the overwhelming majority of the civilian victims in the Syrian conflict have been caused by the use of such indiscriminate aerial weapons."

He said it was "totally unacceptable that the Syrian airforce attacks its own territory in an indiscriminate way, killing its own citizens."

More than 220,000 people have been killed in Syria since March 2011 demonstrations spiralled into a complex civil war.

Several rounds of peace talks have made no headway and the UN envoy's efforts to broker a ceasefire in the second city of Aleppo were rejected by rebel factions.

At least 20 more civilians were killed in regime air strikes Saturday on a rebel-dominated area in the northwestern province of Idlib, the Syrian Observatory for Human Rights said.

"The regime's planes have committed another massacre, this time in Jabal al-Zawiya," the Observatory said, adding many wounded were in critical condition.

After Saturday's barrel bomb attacks on the rebel-held Al-Shaar neighbourhood of Aleppo, victims' bodies were laid out on the streets of the neighbourhood.

The limp blood-covered hand of one of them protruded from under a blanket, said an AFP correspondent at the scene.

Regime barrel bombs -- crude weapons made of containers packed with explosives -- have often struck schools, hospitals, and markets in Syria.

But the toll from the Saturday raids, which also targeted a market in the jihadist-controlled town of Al-Bab, was among the highest.

"This is further shocking proof of the horrific and indiscriminate methods the Assad regime is using to kill and injure innocent civilians, including children," said British Foreign Secretary Philip Hammond.

"Our position remains as strong as ever -- we will continue to call for a political transition to a future in which Assad has no part."

The tactic of carrying out air attacks on built-up areas after battleground losses has become common practise for Syria's regime, which ceded swathes of territory in May.

"The regime has always dropped barrel bombs in this war, but it is intensifying its strikes believing it can compensate for territorial losses," said Observatory head Rami Abdel Rahman.

His group has documented 17,000 barrel bombings by the Syrian regime since October.

"It is also a kind of revenge against civilians who support the rebellion," said Abdel Rahman. "It's a tactic of scorched earth."

On Sunday, a second barrel bomb attack on Al-Bab killed three civilians, while rocket fire on a regime-controlled neighbourhood of Aleppo city killed seven civilians.

Following defeats in Idlib's provincial capital and at a massive military base nearby, government forces also lost the ancient city of Palmyra to IS on May 21.

The jihadists demolished Palmyra's notorious government prison on Saturday that was for decades a symbol of abuses against regime opponents.

IS also seized a major checkpoint at a crossroads south of Palmyra on routes to Damascus as well as Homs to the west, the Observatory said.

IS consolidated its gains in northern Aleppo province, overrunning a village near Turkey, and advanced to within four kilometres of northeast Hasakeh city, which has a large Kurdish population. 

Despite recent losses, Syrian Prime Minister Wael al-Halqi on Sunday pledged the army would "liberate every foot of Syrian territory".

"The army will defeat terrorist organisations and will hold them accountable for their massacres against the innocent," he said, according to Syria's official news agency SANA.

In Iraq, government troops and allied paramilitary forces on Saturday retook an area west of Ramadi, captured by IS two weeks ago.

But Human Rights Watch accused Iraqi authorities of blocking thousands of families from escaping violence in the mainly Sunni province of Anbar.