Washington - The White House sped new humanitarian aid to Syria and heaped fresh pressure on Syrian President Bashar Assad to quit power on Thursday after Kofi Annan resigned as the top international mediator in the bloody 17-month crisis.

In a parting shot, the former U.N. secretary-general and Nobel Peace Prize winner wrote in the Financial Times that the international community had proved "strikingly powerless" in the face of the violence. He also scolded President Barack Obama and his Russian counterpart, Vladimir Putin, saying they needed to show "courage and leadership" to find a compromise.

Obama spokesman Jay Carney told reporters aboard Air Force One that the president was "grateful" to Annan for his work and said his resignation highlighted Assad's determination "to brutally murder his own people" as well as the "failure" of Russia and China to support international efforts to end the bloodshed.