WASHINGTON June 6, 2008  -- The government of Zimbabwe is giving its impoverished citizens a stark choice, U.S. Ambassador James McGee said today: Vote for President Robert Mugabe in the upcoming election — or starve.

Millions rely on food aid in Zimbabwe, but yesterday the regime ordered that foreign aid organizations cease operations. The Zimbabwean government's own food aid programs are now the only source of sustenance for much of the population.

McGee told reporters during a videoconference from the capital, Harare, this morning that his embassy has solid evidence that in order to receive food aid from the government, Zimbabweans must first show their party registration cards.

If they have a card from Mugabe's ruling party they can have access to food, but if they only have opposition cards they must turn over their national identification cards in order to receive the food they need.

The government holds onto the cards until after the June 27 election, McGee says — meaning opposition party members will not be able to identify themselves when they go to vote.

The result, McGee said, is that many in the opposition party are forced to give up their right to vote in exchange for vital food aid.

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

U.S. State Department spokesman Sean McCormack called the move "outrageous." "That is cruel in the most sinister kind of way; using food as a weapon, using the hunger of parents' children against them to prevent them from voting their conscience for a better kind of Zimbabwe," McCormack said. "That's an example of the kind of thing that is going on in Zimbabwe today."

The United States is providing $200 million in aid to Zimbabwe this year, $171 million of which comes as food aid. The balance comes mainly in the form of medical assistance like AIDS prevention.

McGee says the U.S. has no plans to cut that aid, but currently America's help is blocked by the Mugabe government's ban on foreign aid operations.

McGee said that around 1 million people depend on food aid from international sources. He warns there may be "massive, massive starvation" unless the block is lifted. State Department officials estimate that 110,000 people will go hungry this month because they won't have access to food aid provided by only one of the international charities that were distributing food before yesterday's ban.

A professor of political science at the University of Zimbabwe, who spoke on the condition of anonymity because he fears government retaliation, sees another reason for the ban on international aid groups.

"Aid groups being suspended is part of (President Mugabe's party) ZANU-PF's strategy to create a situation … where there [will be] a virtual information blackout in the country. … It is likely that in the event of a complete meltdown, information about massacres and other human rights abuses will be slower in reaching the outside world. Many people in the rural areas are truly desperate and do not know how they will survive from one day to the next," he told ABC News.

At the end of the month Mugabe will face off against his political rival Morgan Tsvangirai in a run-off election. Tsvangirai was detained by police Friday, his second detention this week.

McGee said he fears for Tsvangirai's safety. "Given the excesses of the government here, we are not sure what they will do," he said.

Despite the political violence and intimidation McGee said the run-off should proceed as scheduled on June 27. "Anything less would give the Mugabe regime a victory they do not deserve," he said.

Tsvangirai beat Mugabe in the first round of elections in March, but did not garner enough votes to avoid the run-off.

Mugabe has ruled Zimbabwe since it's independence in 1980, but has resorted to increasingly desperate measure to stay in power in recent years.

McGee called on regional powers, in particular South Africa which has significant leverage with Zimbabwe but has been reluctant to use it, to send in election monitors to ensure as fair an outcome as possible and to protect the voters.

Secretary of State Condoleezza Rice said yesterday that the U.S. isn't currently considering imposing sanctions on Zimbabwe. However, McGee left the door open to other options pending the outcome of the elections.

The United States protested yesterday's harassment of its diplomats on a road north of Harare to Zimbabwe's ambassador in Washington and to Zimbabwe's delegation to a United Nations food conference in Rome.

ABC News' Dana Hughes contributed to this report from Nairobi, Kenya