At least 10 people were killed Friday when air raids targeted rebel-held areas in the city of Aleppo in northern Syria, activists said, threatening an already fragile ceasefire as peace talks falter.

The emergency responder organization known as the White Helmets, which operates in rebel-held areas, said that the fighting in Syria has seriously escalated.

“Today has been the worst day in Syria for over a year,” the humanitarian group said on its Twitter feed.

The International Committee of the Red Cross meanwhile confirmed it was able to deliver aid to Rastan in Homs province, adding that the delivery finished at sunrise, a glimmer of hope as concern mounts for a return to more intense levels of violence.

"We hope this was one of many aid convoys to come," Pawel Krzysiek, the ICRC spokesman in Syria, tweeted.

US President Barack Obama admitted this week while visiting Saudi Arabia that the ceasefire was "under tremendous strain," placing much of the blame on President Bashar al-Assad.

NATO Secretary General Jens Stoltenberg also noted the tensions facing the ceasefire, but cautioned that "it remains the best basis for a negotiated, peaceful solution to the crisis.”

He also noted that Russia maintains a considerable military presence in support of Assad's regime.

The Syrian opposition decided to suspend its participation in the Geneva peace talks this week, citing lack of progress on humanitarian issues and escalating ceasefire breaches.

However, the government's chief negotiator, Bashar Jaafari told reporters Friday that his team would continue talks with UN mediator Staffan de Mistura on Monday in the Swiss UN city.

De Mistura was set to talk to the press in the evening to take stock of the current round of talks that started April 13.

George Sabra, a member of the opposition delegation, which has largely withdrawn from the Swiss city, said the airstrikes in Aleppo were a sign that the decision to pull out of the talks was correct.

"The ceasefire is dead," he told dpa by telephone.

Activists this week were especially alarmed by an airstrike on a market in Maarat al-Numan, in rebel-held Idlib province, which killed about 50 people.

Aleppo remains a key point of concern. Control of the pre-war economic hub is divided between rebels in the east and government forces and militias loyal to President Bashar al-Assad in the west.

There is concern the government plans to impose a siege on the rebel-held sections of the city, where tens of thousands of people live.

"Regime and Russian planes conducted the raids on several rebel-held residential areas," Mahhmoud Shahabi, an activist based inside Aleppo city told dpa by Skype, referring to the raids that killed 10 people.

Activists say al-Qaeda's wing in Syria, al-Nusra Front, has only a small presence inside the city and Islamic State has no formal positions.

Both groups are excluded from the ceasefire brokered by the US and Russia back in February, which initially helped dramatically lower the violence in the country but seems to be losing steam.

Opposition forces have said that Russian strikes largely target rebel positions, especially in northern Syria, with less of a focus on Islamic State.

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