Syria's army seized key ground in its battle to retake Aleppo yesterday, capturing five more districts including a strategic neighbourhood at the heart of rebel territory.
The advance came as Moscow and Washington traded barbs over stalled efforts to end fighting in the city, where forces loyal to President Bashar al-Assad have made significant advances in recent days.
Yesterday, government troops retook five districts including the strategic Shaar neighbourhood and were in control of 70 percent of former rebel territory in east Aleppo, the Syrian Observatory for Human Rights said.
The monitoring group described Shaar as "the most important neighbourhood in the heart of east Aleppo", and said rebels were being reduced to fighting a "war of attrition" with regime troops.
The rapid regime gains have left opposition fighters scrambling to defend the shrinking enclave they still control in Aleppo's southeastern districts.
Despite mounting criticism of the offensive, world powers have struggled to find a way to halt the fighting.
Key Assad ally Russia had announced talks with the United States in Geneva for Tuesday or Wednesday on organising a rebel withdrawal from Aleppo ahead of a ceasefire.
But on Tuesday, Foreign Minister Sergei Lavrov accused Washington, which has backed rebel groups against Assad, of backtracking.
US Secretary of State John Kerry denied any change of plans.
Washington had also accused Moscow of stalling for time after Russia and China blocked a UN Security Council resolution on Monday calling for a seven-day ceasefire.
Syria's foreign ministry said that it would not agree to any ceasefire without a guarantee of a rebel withdrawal.
The rebels have so far rejected any talk of leaving the city, with Yasser al-Youssef of the leading Nureddin al-Zinki faction describing the proposal as "unacceptable".
The loss of Aleppo would be the biggest blow yet to opposition forces in Syria's civil war, which erupted in 2011 with popular protests calling for Assad's ouster.
More than 300,000 people have since died and millions forced from their homes.