Johannesburg:   South Africa's top prosecutor, who is pursuing corruption cases against senior officials including ruling party leader Jacob Zuma, has been sent a letter laced with poison, his spokesman said on Monday.

Mokotedi Mpshe did not touch the letter himself, but National Prosecuting Authority staff who came in contact with the substance in the letter after it was opened were given medical treatment.

Spokesman Tlali Tlali gave no indication who he thought might have sent the letter to the acting head of the prosecuting authority, which has been at the centre of a power struggle between Zuma and President Thabo Mbeki.

"The letter was received on Thursday. It was intended for the national director," said Tlali, adding that some of the staff developed a rash after being exposed to the contents of the letter.

Tlali said an internal investigation had so far not pointed to who could have intended to harm Mpshe. He said the substance did not appear to be extremely dangerous but tests were still underway.

Mpshe became acting head of the prosecutors' office after Mbeki suspended his predecessor. That prompted critics to accuse Mbeki of using the judiciary and state institutions against opponents such as Zuma.

During the apartheid era, police sometimes used letters or items of clothing laced with poison to target opponents. In 1989, they tried to kill African National Congress activist Frank Chikane with poisoned underwear. He is now a top adviser to Mbeki.