After retaking control of about two-thirds of east Aleppo in recent days, forces loyal to President Bashar al-Assad were advancing today on the large residential district of Shaar.
The Syrian Observatory for Human Rights, a monitoring group, said if the district is retaken rebel forces will be reduced to a "war of attrition" with the army.
"It is the most important neighbourhood in the heart of east Aleppo, and is on the brink of falling," Observatory head Rami Abdel Rahman told AFP, adding that regime forces were already in control of about a third of the district.
The regime's rapid gains have left opposition fighters scrambling to defend the shrinking enclave they still control in Aleppo's southeastern districts.
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The international community has also struggled over how to respond, despite widespread concern over the fate of tens of thousands of civilians still in rebel-held areas.
Russia, a key Assad ally, had announced talks with the United States in Geneva for Tuesday or Wednesday on organising a full rebel withdrawal from Aleppo leading to a ceasefire.
"It looks like an attempt to buy time for the rebels to have a breather, take a pause and replenish their reserves," Lavrov told journalists, adding that Moscow had the impression that "a serious discussion with our American partners isn't working out."
Washington for its part accused Moscow of stalling for time after Russia and China blocked a UN Security Council resolution yesterday calling for a seven-day ceasefire.
Russia said the resolution should have been postponed until after the Geneva talks, saying an agreement on organising a withdrawal was close.
"We will not let Russia string along the Security Council," she said.
"We will continue bilateral negotiations (with Russia) to relieve the suffering in Aleppo, but we have not reached a breakthrough because Russia wants to keep its military gains."
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".
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