Twenty more people died during air strikes in Jabal al-Zawiya, a rebel-dominated area in the northwestern province of Idlib, the Syrian Observatory for Human Rights said.

The Observatory called the raids a “massacre” and said many of those left wounded were in critical condition.

Today, 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.

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

The Observatory has documented 17,000 barrel bombings by the Syrian regime since October.

Yesterday, Syrian government troops carried out barrel bomb attacks on the rebel-held Al-Shaar neighbourhood of Aleppo. The 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.

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

“The use of barrel bombs must stop,” UN envoy to Syria Staffan de Mistura 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.”

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.

“It is also a kind of revenge against civilians who support the rebellion,” he said. “It’s a tactic of scorched earth.”

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.

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

Following defeats in Idlib’s provincial capital and at a massive military base nearby, government forces lost the ancient city of Palmyra to the Islamic State 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.

“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,” Hammond said.

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.

Iraqi 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.