Air strikes on rebel-held areas of Syria’s second city Aleppo and a town to its west killed at least 19 people on Tuesday, emergency workers said.

 The strikes came after rebel shelling killed at least 19 civilians in government-held districts of Aleppo on Monday and are the latest in a surge of violence in and around the city that has severely tested a February 27 ceasefire.

Fourteen civilians were killed in the strikes on rebel-held eastern districts of Aleppo city, the civil defence — known as White Helmets — said.

 Five of their own rescue workers were killed when their headquarters in the town of Al-Atarib, controlled by Islamist rebels, was hit by an air strike, the group said on Twitter.

It was not immediately clear whether the strike on Al-Atarib, 35 kilometres (20 miles) from Aleppo, was carried out by the Syrian air force or its Russian ally.

 Fighting has surged on several fronts in Aleppo province, which is criss-crossed with supply routes that are strategic for practically all of Syria’s warring sides.

Once Syria’s commercial hub, Aleppo has been divided between rebel control in the east and government forces in the west since 2012.

 In the rebel-held Fardos neighbourhood, an AFP correspondent saw a youth being helped down a rubble-strewn street with blood streaming from his head and leg.

Violence has rocked the divided city since Friday, with at least 100 civilians killed by artillery or rocket fire and air strikes.

 The fighting severely threatens the February ceasefire brokered by the United States and Russia and comes as UN-brokered peace talks in Geneva stall.

More than 270,000 people have been killed in Syria and millions been forced from their homes since the conflict erupted in 2011.