More than 80 international NGOs and faith groups have issued an urgent plea to world leaders to stop the escalating crisis in Syria.
In a letter to US President Obama and Russian President Putin, the groups warn that "hope is crumbling on your watch".
"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," the letter says.
"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."
The letter refers to "unacceptable violations of Syria's CoH" in the past few days, warning that "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," the letter continues.
"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. 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."
The letter was signed by groups including Christian Aid, CAFOD, Islamic Relief, Oxfam and Tearfund.
It was also signed by the Methodist Church in Britain, the United Reformed Church, the Church of Scotland and Lord Bishop of Coventry, on behalf of the Church of England.
