The president's order in effect overturns a Supreme Court ruling that had been intended to relax prison conditions for the Americans Gary Blanchard, Joseph Pettijon and John Dixon.

It is the second time this year that Mr Mugabe has expressed views contrary to Supreme Court decisions.

President Mugabe criticised a court ruling on two detained journalists

In February, he challenged four judges to resign after they asked him to comment on the illegal detention and alleged torture of two journalists.

Reports in the government-controlled newspaper The Herald said the president's move had given the chief prisons administrator the right to decide how prisoners were kept.

Justice Minister Emmerson Mnangagwa said in a statement that prison security was a matter of executive discretion which the judiciary had "usurped".

The Supreme Court had earlier ordered that the three prisoners should be allowed contact with one another inside the prison.

'Security risk'

President Mugabe: Put the decision in the hands of prison authorities

The minister said "such an order would result in chaos in the running of prisons if all prisoners were to decide how and where they should be kept and who their cellmates should be."

He said the three men were a security risk because sophisticated arms of war had been found in their possession.

The three prisoners have complained of being subjected to inhuman treatment during their four-month detention, including solitary confinement and torture by electric shock.

The Americans were arrested at Harare airport in March, and have been accused of possessing arms destined for rebels in the Democratic Republic of Congo.