The UN Security Council met on Wednesday for crisis talks on Syria as air raids shook Aleppo and diplomatic tensions ran high over an attack on an aid convoy.
UN Secretary-General Ban Ki-moon told the council that a ceasefire ended by the Syrian army must be revived as a first step to salvage a US-Russian deal on ending the five-year war.
"We are at a make or break moment", Ban told the council.
The UN chief urged world powers to use their influence to help re-start political talks between Syrians and allow them "to negotiate a way out of the hell in which they are trapped."
US Secretary of State John Kerry and Russian Foreign Minister Sergei Lavrov were taking part in the tense meeting after Washington accused Moscow of being responsible for the attack on the aid convoy.
Ban told the council that he was "looking at options for vigorously investigating this and similar atrocities against civilians" during Syria's war.
Intense air raids shook Syria's Aleppo and killed a team of medics on Wednesday, with bombing escalating since the Syrian military declared an end to the week-long truce on Monday.
Russia and the United States negotiated the ceasefire plan, with Kerry warning it could be the "last chance" to try to end war, which has killed more than 300,000 people in five years.
"We must remain determined that the ceasefire will be revived," Ban said.
"The Syrian tragedy shames us all. The collective failure of the international community should haunt every member of this council," he said.