(Adds more from State Department spokesman)

By Sue Pleming

WASHINGTON, June 6 (Reuters) - The United States on Friday accused President Robert Mugabe's government of using food aid as a political weapon to get votes in Zimbabwe's run-off election on June 27.

"We are dealing with a desperate regime here which will do anything to stay in power," said U.S. Ambassador to Zimbabwe James McGee in a video call from Harare, one day after Zimbabwean police held five U.S. and two British diplomats.

McGee told Washington reporters if potential voters wanted food aid they had to show their voting cards, which indicated whether they belonged to Mugabe's ruling ZANU-PF party or the opposition Movement for Democratic Change.

"If you have an MDC card, you can receive food but first you have to give the national identity card to the government officials, which means they will hold onto it until after the election. Again, you will not be able to vote," he said.

Supporters of the ruling party could keep their identity cards and so were free to vote in the election. Mugabe has been in power since Zimbabwe's independence from Britain in 1980.

"The only way you can access food is to give up your right to vote," said McGee. "It is absolutely illegal."

About 4 million Zimbabweans rely on food aid in a country once seen as the region's breadbasket but where inflation is now a staggering 165,000 percent and unemployment 80 percent.

On Thursday, Mugabe's government suspended the work of all international aid agencies in the southern African country, saying some of them were campaigning for the opposition.

But U.S. State Department spokesman Sean McCormack said foreign aid groups were being kicked out because the Mugabe government wanted to be the sole source of food aid.

"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 told reporters.

FOOD RUNNING OUT

McGee also condemned the action and said while there appeared to be enough food in stock until the election, he predicted "massive, massive starvation" after the poll.

The United States is furious at Zimbabwe over its treatment on Thursday of five U.S. and two British diplomats who were detained for several hours. U.S. Secretary of State Condoleezza Rice called the incident "outrageous behavior."

Washington raised the issue at the U.N. Security Council and protested strongly to Zimbabwe's government, saying it underlined the kind of harassment Zimbabweans faced daily.

On Friday, Zimbabwean opposition leader Morgan Tsvangirai was detained by police for the second time this week after blocking him from reaching a campaign rally for the run-off.

Tsvangirai beat Mugabe in a March 29 election but failed to win the majority needed to avoid a second ballot. His party says 65 people have been killed in violence since the first round of voting. Mugabe says the opposition is responsible.

McGee said given the "excesses" of the government, the United States was unsure what it would do next to Tsvangirai.

"We in the diplomatic community want to assure the entire world that we are keeping a light on the activities of the government in Zimbabwe and hopefully that would deter them from doing anything that might be detrimental to Mr. Tsvangirai," said McGee.

The United States has already imposed rigid travel and other sanctions on Mugabe and officials in his party and McGee hinted of more targeted, punitive measures in the future.

"If, as so many people are theorizing, ... this election is stolen by the Mugabe regime, then we will take a look at what our response might be," he said. (Additional reporting by Arshad Mohammed, editing by David Alexander and Eric Beech)

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