The government of Robert Mugabe has suspended all field work in Zimbabwe by aid groups and non-governmental organizations, accusing them of not operating under the terms of their government registration.

Officials claim some of the groups have been campaigning for the opposition in this month's upcoming presidential runoff election. The agencies deny the charge and some speculate the government does not want them to witness intimidation of opposition supporters.

Earlier on Thursday, police and a mob of "war veterans," often violent Mugabe loyalists, detained and assaulted a group of U.S. and British diplomats for several hours at a roadblock north of the capital Harare, a U.S. official said.

State Department spokesman Sean McCormack said the detention and harassment of the U.S. diplomats is "absolutely outrageous" and the U.S. plans to raise the issue with the UN Security Council as well as Zimbabwean officials attending a UN food conference in Rome.

The incident is indicative of "repression and violence" Zimbabwe's government is willing to use against its own people, said McCormack.

Officials said five Americans, four Britons and three Zimbabweans in a three-vehicle convoy were attacked while trying to investigate political violence in the country.

Zimbabwean police stopped the vehicles and slashed their tires, U.S. Ambassador James McGee told CNN earlier in the day by telephone from Harare.

Officers also "reached in and grabbed telephones from my personnel" while the war veterans threatened to burn the vehicle with the foreign officials inside if they didn't accompany the police to a nearby station, he said.

A spokesman for the U.S. Embassy, Paul Engelstad, also said a U.S. staffer was beaten.

The U.S. officials were detained for about six hours about 45 kilometres north of Harare, McGee said.

"The government is trying to intimidate its own people into voting in what they would consider the correct way, and now it's trying to intimidate diplomats from travelling to the countryside to witness the violence that's been perpetrated against the population here in Zimbabwe," McGee said.

"We do believe this is coming directly from the top," he added.

'It's a very sad situation'

Police spokesman Wayne Bvudzijena denied security agents had threatened the diplomats, saying instead that police were trying to rescue them from a threatening mob.

"It's unfortunate when diplomats behave like criminals and distort information," Bvudzijena said. "It is a very sad situation."

The opposition party and rights groups have accused Mugabe of orchestrating violence and intimidation in the run-up to a June 27 presidential run-off.

On Wednesday, opposition party leader Morgan Tsvangirai was detained by police for more than eight hours after he was stopped at a roadblock while campaigning for the upcoming run-off election.

The second round of voting comes after neither the opposition nor longtime President Mugabe gained the simple majority needed to avoid a run-off, according to official election results.

Tsvangirai, leader of the Movement for Democratic Change, claimed outright victory after the March 29 vote.

With files from the Associated Press