Heavy Syrian government bombing killed at least 27 civilians on Saturday, activists said, as the latest bout of violence hit the war-ravaged country.

Twelve civilians were killed in an air strike on the northern metropolis of Aleppo on Saturday, a local civil defence official said. The Syrian Human Rights Observatory said a further 13 people died in shelling on the rebel-held town of Douma, east of Damascus, while two men were killed in government air strikes on Talbisseh in central Homs province.

Air strikes on Aleppo targeted several neighbourhoods, including the heavily populated Bustan al-Qasr district, an AFP correspondent in the city said. The deadliest raid was on the Tariq al-Bab neighbourhood on the eastern edges of the city.

A truce brokered by Russia and the United States had raised hopes that UN-backed talks in Geneva this month would lead to a solution to the five-year conflict. But the negotiations, due to continue until Wednesday, have faltered after Syria's main opposition group this week suspended its official participation in the talks.

After the suspension, Staffan de Mistura, the UN special envoy, said the fragile truce was “in great trouble if we don’t act quickly,” although he stressed that his team had continued to meet remaining members of the opposition High Negotiations Committee at their Geneva hotel.

It was the second day of deadly strikes on Aleppo, after 25 civilians were killed and another 40 wounded in air strikes on Friday.

Observatory head Rami Abdulrahman said the escalating violence meant the ceasefire between the government and most rebel groups in place since late February had effectively collapsed.

"Most of the areas that were under the ceasefire are now seeing fighting again," he said.

A ceasefire has not been in force with groups designated terrorist organisations by the UN, including the Islamic State and al-Qaeda’s Syria affiliate the Nusra Front.

"The ceasefire ended when the first bomb hit the city," Muhammed Mashhad, a civil defence volunteer told AFP. "The regime is intensifying its air strikes, which have reached around 20 a day.

"This regime is criminal and doesn’t understand the language of political negotiations. All it gets is bombing, killing and destruction," the 42-year-old said.

The government has always maintained that it is fighting terrorists.

The uptick in violence in Aleppo came as US Secretary of State John Kerry on Friday warned that Russia had moved heavy artillery into position outside the city, and he questioned Russian President Vladimir Putin's sincerity in claims that the build-up was to combat terrorism.

“We are not going to sit there and let him do his thing, supporting the regime and hammer at the opposition and say, ‘This is working,’” Kerry told the New York Times. “Obviously, we’re not stupid about it.”