BEIRUT, Lebanon — Russia escalated its anti-American invective Thursday in the deadlocked diplomacy over the Syrian war, dismissing a threat by Secretary of State John Kerry to halt talks and accusing his spokesman of abetting global terrorism — including against Russian military personnel in Syria.

The Russian response came as United Nations officials warned that 600 wounded civilians in the rebel-held districts of the divided northern city of Aleppo must be evacuated and that food is nearly exhausted for the 275,000 residents trapped there.

The residents have been pummeled for the past week by Syrian and Russian airstrikes, including by powerful bombs that can pulverize underground shelters. The top emergency relief official for the United Nations, Stephen O’Brien, told the Security Council on Thursday that Aleppo faced a “humanitarian catastrophe unlike any we have witnessed in Syria.”

The Russians, the main military allies of President Bashar al-Assad, offered 48-hour pauses in Aleppo to permit humanitarian access, an idea that Western diplomats and United Nations officials have rejected as impractical and meaningless. Mr. Kerry has demanded a return to the cessation of hostilities agreement he negotiated with Foreign Minister Sergey V. Lavrov on Sept. 9, which collapsed a week later.