Heavy bombardment kills 25 in Syria's Aleppo province

A barrage of airstrikes and shelling on the northern Syrian city of Aleppo and a town to its west have killed 25 civilians, emergency workers and a monitoring group said Tuesday.

The attacks are the latest in a surge of violence in and around the city that has severely tested a February 27 ceasefire.

At least two male civilians died in rebel rocket fire on government-controlled areas in the west of the city on Tuesday afternoon, said the Syrian Observatory for Human Rights.

In the rebel-held eastern parts, the air strikes and shelling came down "like rain", one resident told AFP.

Fifteen civilians were killed in airstrikes on several rebel-held eastern districts of Aleppo city, the civil defence, known as White Helmets, said.

Another three civilians - two women and a child - were killed in government artillery shelling on another eastern neighbourhood, they said.

"The planes are bombing markets, residential areas... We're exhausted, we can't keep up," a civil defence worker said.

Five of their own were killed when the White Helmets headquarters in the town of Al-Atarib, controlled by Islamist rebels, was hit by an overnight 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.

An ambulance and a fire truck, both damaged, were parked in the bombed-out headquarters, surrounded by rubble and twisted metal frames.

A civil defence worker in Aleppo city said he and his colleagues were afraid their local headquarters would also be targeted.

Fighting has surged on several fronts in Aleppo province, which is crisscrossed 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.

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

On Monday, rebel shelling killed at least 19 civilians in government-held districts, the Observatory said.

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

Syria's main opposition group, the High Negotiations Committee (HNC) halted its formal participation this week in the Geneva talks, which started on April 13.

A leading opposition group, the National Coalition, condemned the strike on Al-Atarib and hailed the "remarkable efforts and bravery of Civil Defence workers".

"Favourable conditions for the political process cannot be created whilst the Assad regime's killing machine continues to wreak death across Syria," the Coalition said in an online statement.

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

Agencies contributed to this report.