U.S.: Zimbabwe could see 'massive starvation'

Tsvangirai, who lost a 2002 presidential election that independent observers said was rigged in Mugabe's favor, had only returned to Zimbabwe in late May to campaign for the runoff. He left the country soon after the March first round, and his party has said he was the target of a military assassination plot.

The Movement for Democratic Change says at least 60 of its supporters have been slain in the past two months.

On Thursday, a mob of Zimbabwe "war veterans," a group of often violent Mugabe loyalists, waylaid a convoy of American and British diplomats investigating political violence, beating a local staffer, slashing tires and threatening to burn the envoys, the U.S. Embassy said.