Air strikes in Syria's Aleppo kill at least 25 civilians

Air strikes and shelling in Aleppo and a town to its west left 25 civilians reported dead on Tuesday, as a surge in violence tests a troubled ceasefire.

UN chief Ban Ki-moon said he was "deeply concerned" by the fighting and urged both sides to stick to the two-month-old truce and troubled peace talks in Geneva.

"The cessation of hostilities should go on, otherwise it will be very difficult for humanitarian workers to deliver," Ban told reporters in Vienna.

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

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

Fifteen civilians were killed in air strikes on several rebel-held districts, according to civil defence volunteers known as the White Helmets.

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," one civil defence worker said.

Five of their own were killed when the White Helmets headquarters in the town of al-Atarib, controlled by rebels, was hit by an overnight air strike, the group said on Twitter.

It was not immediately clear whether Russia or President Bashar al-Assad’s air force carried out the strike on al-Atarib, 35 kilometres from Aleppo.

At least 19 civilians were reported killed and 120 wounded on Monday in a rebel bombardment of Syrian government-held districts of the northern city of Aleppo, a monitoring group said.

Rebel-held neighbourhoods in eastern Aleppo have had their water and electricity supplies cut by bombardment, an AFP journalist said.

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

"Shells fired ... by rebel groups at districts under regime control left 19 dead, including three children, and 120 wounded," Rami Abdel Rahman, the head of the UK-based Syrian Observatory for Human Rights, told the AFP news agency.

Syria's SANA state news agency reported 16 dead and 86 wounded by "fire from the terrorist groups Nusra Front [al-Qaeda's affiliate in Syria] and its allies" on at least five residential districts.

The Observatory also reported four people including a child killed by government fire targeting rebel-held neighbourhoods of the city.

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 latest round of Geneva talks that started on 13 April.

UN Syria envoy Staffan de Mistura is due to give a progress report to the UN Security Council on Wednesday, when the talks are scheduled to go into recess.

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.