(CNN) -- Millions of people in Zimbabwe already facing economic hardship and hunger are being put at risk by a government ban on relief organizations, the United Nations warned Friday, saying it would urge a lifting of restrictions.
Robert Mugabe's supporters are accused of mounting a campaign of intimidation and violence.
Agostinho Zacarias, the U.N.'s top humanitarian coodinator in Zimbabwe, met senior government figures Friday to try to get the ban overturned and allow aid agencies resume providing food, clean water, medical care and other services.
"The government said they didn't take this measure to starve the people. They have their reasons. The officials we discussed this with were not prepared to advance any reasons," he said.
Zacarias said the ban was likely to affect millions of people.
He also met with church leaders to explore aid alternatives as they don't fall under the category of aid groups hit by the ban.
Bright Matonga, deputy information minister for Zimbabwe, accused several non-governmental organizations of telling people they would not receive food unless they voted for an opposition presidential candidate.
The U.S. ambassador to Zimbabwe, James McGee, said Zimbabwean authorities were using food as a weapon to intimidate the population and hold on to power. Watch how international outrage is growing »
"What we have is a bunch of greedy people who want to remain in power at all costs," McGee said from Zimbabwe's capital, Harare.
McGee said people seeking food from the government are forced to give up their identity cards if they are not registered as supporters of the government, meaning they will be unable to vote.
In another development Friday, Zimbabwe opposition leader Morgan Tsvangirai was arrested for the second time this week, his spokesman said.
Tsvangirai was stopped at a roadblock and taken to a police station as he was on his way to a regularly scheduled rally, his spokesman George Sibotshiwe said. He was released after 2½ hours.
"We've noticed that it's going to be a common trend in this campaign, and obviously the government and Robert Mugabe are trying to prevent [Tsvangirai] from going about his campaign freely and peacefully," Sibotshiwe said.
He said that unless the African Union deploys peacekeepers to the country, "campaigning in Zimbabwe is now virtually impossible."
"What I can convey is that since this morning we have had 10 or 11 central intelligence organization vehicles following us everywhere. There was heavy intimidation, with armed military people following us everywhere as well, and they basically pushed the president up to this roadblock before arresting him."
Sibotshiwe said there were no grounds for the arrest. "The way they work here is they don't give you any reason," he said. "Obviously, there is no charge."
The aid ban was made public Thursday, but CNN has obtained a memo dated Wednesday in which Zimbabwe's social welfare minister, Nicholas Goche, told non-governmental organizations "to suspend all field operation until further notice."
It said "a number of NGOs involved in humanitarian operations are breaching the terms and conditions of their registration."
Zimbabwe accuses international aid groups of political meddling ahead of a June 27 runoff that opposition groups say longtime President Robert Mugabe is trying to rig through intimidation.
Matonga said agencies must re-register with the government and state their purpose clearly to continue working in Zimbabwe and the government hopes that happens soon.
Kenneth Walker, a spokesman for the aid agency CARE, said Friday that the government's action has sowed confusion.
"All the NGOs are in the dark. They have no idea what this letter means. They have no idea how long it's going to last," he said.
"There's some serious concern about the impact on the millions of Zimbabweans who now won't be receiving food aid, clean water and sanitation facilities, help with agriculture and a wide variety of other services that the NGOs provide."
Henrietta Fore, administrator of the U.S. Agency for International Development, urged the government of Zimbabwe Thursday to "lift the suspension on all international aid agencies involved in humanitarian work in the country."
Fore said the "suspension is a direct threat to the lives and well-being of tens of thousands of innocent people in Zimbabwe."