Zimbabwe's government says it has indefinitely suspended the work of all aid groups in the country, accusing them of violating the terms of their registration.

Zimbabwe's social welfare minister, Nicholas Goche, said Thursday he has learned that aid and development groups are breaching the terms of their agreement with government. He did not elaborate.

Earlier this week, the aid group CARE said it was ordered to suspend its work because of alleged political activity. The group denies the charge.

Human Rights Watch accuses Zimbabwe's government of using food as a political weapon ahead of a June 27 presidential run-off election between President Robert Mugabe and opposition lead Morgan Tsvangirai.

Also Thursday, the United States expressed outrage after Zimbabwean security forces detained for several hours U.S. and British diplomats trying to visit victims of political violence.

Secretary of State Condoleezza Rice called the situation grave and said the United States will raise the matter with the U.N. Security Council.

U.S. Ambassador to Zimbabwe James McGee says police stopped the diplomats at a roadblock 45 kilometers north of the capital and slashed tires on their vehicles. He says militant supporters of President Mugabe threatened to burn the diplomats in their cars if they did not go with police to a local station.

Zimbabwean officials reject McGee's account, saying police were trying to protect the diplomats from an unruly mob.

In London, Britain says it has summoned Zimbabwe's ambassador to explain the incident.

Security forces detained Mr. Mugabe's challenger in the presidential race, Mr. Tsvangirai, for several hours Wednesday before releasing him. In a statement Thursday, Mr. Tsvangirai said his treatment demonstrates the lengths Mr. Mugabe will go - in his words - to "try and steal" the June 27 run-off.

Mr. Tsvangirai won the first round of the presidential election on March 29, although the official tally shows that he did not win enough votes to avoid a run-off.

Some information for this report was provided by AFP, AP and Reuters.