Lavrov charged, and administration officials sharply denied, that the United States had "revoked" the Friday evacuation proposal. "Serious conversations with our partners do not work," he said, adding that the United States had notified Moscow that it would not attend a new meeting on the plan.
Lavrov said a U.S.-backed U.N. Security Council resolution over the weekend calling for a seven-day Aleppo truce was proof that other U.S. officials had "disavowed" Kerry's efforts. Russia and China vetoed the resolution. They chose to do so, Britain's U.N. Ambassador Matthew Rycroft said, "because of their long-standing, misplaced faith in a despot who has killed nearly half a million of his own people, who has sanctioned the murder of civilians as they flee the bombed-out ruins of Aleppo."
Syrian President Bashar al-Assad, Rycroft said, "would rather reduce Syria to rubble than to negotiate an overdue peace."
Meanwhile, Kerry told reporters here that he was "not aware of any specific refusal" to meet with Lavrov. During a stop Monday in Berlin, he said the two would meet Thursday in the German city of Hamburg where they will attend a meeting of the Organization for Security and Cooperation in Europe.
Administration officials, speaking on the condition of anonymity, suggested that Lavrov was trying to cause mischief and avoid anything that would end the civil war while Russia and Syria destroy as much of Assad's opposition as possible. A brutal government offensive over the past week has driven the opposition out of much of the territory in eastern Aleppo it has held since 2012, and the city is thought to be just days away from falling.
The U.S. goal, the officials said, is to save as many lives as possible before that happens. They said it had been clear from the start of the most recent Kerry-Lavrov talks that the rebels would have to be consulted on a departure plan. Those discussions, the officials said, are ongoing.
Syrian government forces and allied militias captured Aleppo's centrally located al-Shaar neighborhood from rebels Tuesday, securing nearly three quarters of the besieged enclave less than two weeks after launching a ground offensive, according to the Syrian military.
Rebels withdrew from al-Shaar under heavy bombardment by pro-government forces to the Marjeh and Maadi neighborhoods, local media activist Mahmoud Raslan told The Associated Press.
"Morale has hit rock bottom," he said from inside the city's remaining rebel-held enclave.
The SANA state news agency said the government captured the entire neighborhood as well as the neighborhoods of al-Qatarji and Karm al-Dada.
Al-Shaar was home to at least four hospitals available to residents trapped by the government's siege of the eastern part of the city. But those hospitals, along the rest of the neighborhood, were bombed by the government's attacks and evacuated.
Rebel leaders acknowledged that they were discussing an evacuation with the United States but said they had yet to be presented with a comprehensive proposal.
"The U.S. and Russia couldn't agree, so there was no plan put up for acceptance," said one person close to the opposition who spoke on the condition of anonymity.
The evacuation plan — under which U.S.-backed rebels and civilians would depart the city under secure conditions, leaving only the forces of the al-Qaida-linked group formerly known as Jabhat al-Nusra — is different from previous plans.