Overnight air strikes and a rocket attack on an opposition-held area west of Aleppo killed five rescue workers, who appeared to have been deliberately targeted, a monitoring group and colleagues nearby said, according to Reuters.

The raids, which the rescue workers said consisted of two air strikes and at least one rocket, hit a centre for the Syrian Civil Defense, known as the "White Helmets", in the town of Atareb, some 25 km west of Aleppo.

Fighting has intensified in Aleppo province in recent weeks as a partial truce brokered the United States and Russia has all but broken down.

The Civil Defense corps work as first responders in opposition-held territory where medical infrastructure has broken down.

"The targeting was very precise," Radi Saad, a Civil Defence worker based in northwestern Syria, told Reuters via internet.

"They were in the centre and ready to respond. When they heard warplanes in the area they did not think they would be the target."

It was unclear whether Syrian or Russian warplanes had launched the raids, he said. There was no immediate comment from the Syrian government.

Another Civil Defence member, Ahmad Sheikho, said five rescue workers had died and two were seriously wounded. Ambulances and cars belonging to doctors were also destroyed, Reuters said.