UN Secretary-General Ban Ki-moon on Monday urged Russia and the United States to put Syria's ceasefire back on track and stressed that new truce arrangements in place for two areas must be extended to Aleppo.
The comments came as heavy air strikes hit rebel-held east Aleppo in the early hours of Monday, days after the United States and Russia announced plans to reinforce the February 27 truce in Latakia and Damascus regions.
Ban is "profoundly concerned about the dangerous escalation of fighting in and around Aleppo and the intolerable suffering, counted in mounting deaths and destruction, it is causing among civilians," said UN spokesman Stephane Dujarric.
The UN chief noted the re-launch of the cessation of hostilities in Damascus and Latakia and stressed "the need to expand these arrangements to other parts of Syria, with a special urgency for Aleppo."
The appeal came on the eve of talks between Ban's envoy for Syria, Staffan de Mistura, and Russian Foreign Minister Sergei Lavrov in Moscow on the collapsing ceasefire.
He called on all international actors, in particular the United States and Russia "to redouble their efforts in support of the Syrian parties to put the cessation of hostilities back on track."
"The collapse of the cessation of hostilities will only bring more violence, death and destruction while further weakening efforts to find a negotiated solution to this brutal war," warned Ban's spokesman.
Earlier on Monday, U.S. Secretary of State John Kerry warned that Syria's civil war is "in many ways out of control", as he vowed to work hard in the "coming hours" to salvage the tattered truce.
More than 270,000 people have been killed since Syria's conflict erupted in March 2011 with protests demanding that leader Bashar al-Assad step down.
AFP contributed to this report.