On Friday rebels reportedly shelled a mosque in a government-held neighbourhood of Aleppo, killing at least 15 people as they left Friday prayers.
The attack came as government air strikes hit rebel-held parts of the city, killing at least six people, even after the Syrian army unilaterally declared a brief truce in other parts of the country.
The violence in Aleppo has killed more than 200 civilians over the past week and is likely to continue unchecked, as the government's cease-fire does not include Aleppo.
85 faith groups and non-governmental organisations are now asking President Putin and President Obama to work together to ensure that the ceasefire in the country is stuck to.
Christian Aid, CAFOD, Church of Scotland and Tearfund are just some of the names of the list that also includes Methodists and Baptists.
They have released a statement that reads: "Violence across Syria has escalated alarmingly, reportedly claiming a life on average every 25 minutes in the past 48 hours.
"We cannot stand by in the face of this catastrophe. Global NGOs and faith groups demand that President Barack Obama and President Vladimir Putin immediately use their personal diplomatic engagement to save what remains of the Cessation of Hostilities (CoH) agreement.
"In recent days, unacceptable violations of Syria's CoH and the rules of international law have threatened to doom the initiative in its entirety.
"Numerous strikes on civilians, including several targeting medical facilities and rescue workers, have killed dozens.
"This must stop now. Just weeks ago, the US and Russia helped secure the CoH, providing a glimpse of what a respite from violence means for people in Syria.
"Fewer people died in the first month of Syria's ceasefire than at almost any other time since the war began. For many, it was the first time in years that they could walk the streets without fear.
"This was a first glimpse of hope. That hope that now hangs in the balance."
It went on: "Syria is yet again on the brink of humanitarian disaster and the need for action is urgent.
"President Obama and President Putin: these atrocities are happening on your watch.
"Please heed the call of the UN Special Envoy to Syria to take urgent steps to rescue Syria's cessation of hostilities and end attacks on civilians. Please act now to keep hope alive for Syrians."
Some signatories:
- Action on Armed Violence
- Baptists Together
- CARE International
- Hand in Hand for Syria
- Islamic Relief
- Methodist Church in Britain
- Oxfam
- Relief & Reconciliation for Syria
Civil war broke out in Syria in December 2012 between government-backed forces and rebels opposed to the rule of Preisdent Bashar al-Assad.
Unrest began in the early spring as 2011, during the Arab Spring protests, with nationwide protests against President Bashar al-Assad's government, whose forces responded with force.
The conflict gradually changed from mass protests to an armed rebellion.
