By Jon Herskovitz

JOHANNESBURG (Reuters) - A South African court ruled on Tuesday its legal system can be used to investigate and prosecute citizens of neighboring Zimbabwe suspected of crimes against humanity.

The decision by a high court in Pretoria could prod South Africa into launching investigations into high-ranking Harare officials that would strain already difficult diplomatic relations with the power-sharing government in Zimbabwe.

The ruling applies to Zimbabweans in South Africa and could be extended to those who have travel plans to the country.

President Robert Mugabe and several members of his ruling ZANU-PF have already been hit with international sanctions for suspected human rights abuses that include using death squads to intimidate citizens and rigging elections to stay in power.

"This judgment will send a shiver down the spines of Zimbabwean officials who believed that they would never be held to account for their crimes but now face investigation by the South African authorities," said Nicole Fritz, executive director of the Southern Africa Litigation Centre - one of the applicants.

Several ZANU-PF officials face travel bans to many Western countries and the court decision could make it difficult for them to visit South Africa where they could face criminal prosecution.

Judge Hans Fabricius said South Africa was bound by its international legal responsibilities to investigate Zimbabwean officials linked to acts of state-sanctioned torture.

"We have noted the judgment. We will decide which legal avenues to explore," said Mthunzi Mhaga, spokesman for South Africa's National Prosecuting Authority.

Mugabe's ZANU-PF has avoided criminal investigations at home because it controls state security and court system. It has been accused of suppressing opposition through strong-arm police tactics and prosecuting its foes on trumped-up charges.

In power since independence from Britain in 1980, Mugabe says the allegations of abuses are part of an international plot to unseat him.

South Africa allowed more than a million people from Zimbabwe to enter without documents three years ago when its destitute neighbor was swept up in political violence and its already unsteady economy was being crushed by hyperinflation.

Many thousands of these say they are victims of political violence, and could press prosecutors to investigate rights abuses.

(Editing by Alison Williams)