The U.S. ambassador to Zimbabwe said Friday that the government of President Robert Mugabe is using food as a weapon to try to retain power in the presidential run-off election scheduled for June 27.

Ambassador James McGee also expressed concern about the safety of opposition leader Morgan Tsvangirai.

In a videoconference to reporters in Washington from Harare, McGee said that if the situation continues, "massive, massive starvation" will result.

However, McGee called for the runoff to precede, despite signs that the government was trying to undermine a fair vote with violence and coercion.

"Anything less than a runoff would just be giving the Mugabe regime a victory they do not deserve," he said.

On Thursday, Zimbabwe's government ordered aid groups to halt operations in a move that could hamper food deliveries in the impoverished nation where millions depend on outside help.

The United States says that was an attempt to make the government the sole distributor of food.

McGee said the government is now supplying food mostly to its supporters. He said the U.S. embassy has evidence that the government is offering food to opposition members only if they turn in identification that would allow them to vote.

"It's just another despicable act in a litany of despicable acts committed by this government against its own people," State Department spokesman Sean McCormack said Friday.

McGee expressed concern for opposition candidate Tsvangirai, who was detained briefly Friday for the second time this week and prevented from campaigning.

"Do I fear for Morgan Tsvangirai's life? Given the excesses of the government here, we are not sure what they will do," he said.

Tsvangirai beat Mugabe in the March 29 first round, but did not garner the 50 percent plus one vote necessary to avoid a runoff.

McGee said he believes that the government is desperate.

"What we have is a bunch of greedy people, who want to stay in power at any cost," he said.

Opposition and human rights groups accuse Mugabe of orchestrating violence to ensure he wins re-election amid growing unpopularity for his heavy-handed rule and the country's economic collapse.

On Thursday, a mob believed loyal to Mugabe detained, then released, a convoy of American and British diplomats near the Zimbabwean capital.

McGee said the diplomats were trying to monitor violence against opposition members in the countryside.

Mugabe frequently accuses Britain and the United States of plotting to topple him and return Zimbabwe to colonial rule.