Aid agencies have expressed deep concern about Zimbabwe's order for humanitarian groups to suspend work, an order that sparked fears that food might be used by the government as a political weapon. The US ambassador to Zimbabwe said the government was trying to become the sole distributor of food to help President Robert Mugabe stay in power. Opposition leader Morgan Tsvangirai will challenge Mugabe in the June 27 presidential runoff. Ambassador James McGee said the government is supplying food primarily to its supporters. He said the US embassy has evidence the government is offering food to opposition members only if they turn in identification that would allow them to vote. "This is further punishment of the population of Zimbabwe that will single out people who are not Mugabe supporters," said Carolyn Norris, deputy director of the Africa division with Human Rights Watch. "It puts food distribution firmly in hands of government," she said. Britain's foreign aid chief called the decision to restrict humanitarian agencies' work "indefensible," and said it showed "the lengths to which Mugabe will go to cling to power."