(REUTERS) – U.S. Secretary of State John Kerry accused Syrian President Bashar al-Assad’s government of carrying out “nothing short of a massacre” in Aleppo, where thousands on Thursday were evacuated under a ceasefire deal from the last rebel enclave in a city besieged by fighting for years.

Kerry vigorously defended U.S. diplomatic efforts to end the war – all of which have been futile – and in which Assad, backed by Russia, Iran and Shi’ite militias, has gained the upper hand in the latest turn of the nearly six-year-old conflict.

Ultimately, Washington was forced to watch from the sidelines as the Syrian government and its allies mounted an assault to pin down the rebels in a steadily shrinking pocket of territory in eastern Aleppo, culminating in this week’s ceasefire deal.

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