The UN Security Council has been warned not to allow civilians on both sides of the Syrian city of Aleppo to be cut off from humanitarian aid.
Insurgents effectively broke a month-long government siege of eastern, opposition-held Aleppo on Saturday, severing the primary government supply corridor and raising the prospect that government-held western side of the city might become besieged.
The US, Britain, France, New Zealand and Ukraine organised an informal Security Council meeting on Aleppo on Monday with briefings by a "White Helmet" rescue worker along with two US-based doctors from the Syrian American Medical Society who recently returned from the city.
"If the fighting continues it is conceivable that civilians on both sides of Aleppo could be cut off from the basic assistance they need. We cannot allow this to happen," US Ambassador to the United Nations Samantha Power said.
Citing UN figures, Power said Syrian government forces were to blame for nearly 80 percent of the besieged areas throughout Syria. Aleppo, Syria's biggest city before the outbreak of the conflict five years ago, has been divided between government forces and rebels since the summer of 2012.
"We once again urge Russia to stop facilitating these sieges and to use its influence to press the regime to end its sieges across Syria once and for all," she said.
The United Nations aid chief has called for weekly 48-hour humanitarian pauses in fighting to deliver aid to Aleppo.
Russian Deputy UN Ambassador Vladimir Safronkov accused the United States and its western colleagues of politicising a humanitarian issue, urging them to "admit that the main cause of all of the humanitarian problems in Syria is not the counter-terrorist actions by the legitimate government of Syria.
"The propaganda and the emotional rhetoric, the unfounded accusations, the information campaign, means that we cannot move toward a political settlement in Syria," Safronkov said.
He said the first step toward ending the five-year conflict should be a pooling of efforts to combat terrorism and then a renewal of Syrian peace talks.
A crackdown by Syrian President Bashar al-Assad on pro-democracy protesters five years ago sparked a civil war, and Islamic State militants have used the chaos to seize territory in Syria and Iraq.
The United States and allies began bombing Islamic State militants in Syria nearly two years ago, while Russia began air strikes in support of Assad a year ago.