Zimbabwe ? A mob believed loyal to President Robert Mugabe assaulted a convoy of U.S. and British diplomats Thursday, and the government ordered aid groups to halt operations in a move that could hamper food deliveries in impoverished Zimbabwe.

The U.S. Embassy said a group of Zimbabwe “war veterans,” often violent Mugabe loyalists, waylaid a convoy of American and British diplomats Thursday, beating a local staffer, slashing tires and threatening to burn the envoys.

The diplomats were looking into political violence before a presidential election runoff, and the incident was the latest sign of how tense Zimbabwe is as Mugabe prepares to face an opposition leader who led voting in the first round.