South African police have put a "red alert" on the country's borders to prevent any attempted flight by Zimbabwean first lady Grace Mugabe, who is accused of assaulting a model in a Johannesburg hotel with an electric cable.

Her 93-year-old husband, President Robert Mugabe, is also in South Africa for a regional summit, complicating the diplomatic dilemma confronting Pretoria, which already has a difficult relationship with its troubled northern neighbour.

South Africa is home to an estimated three million Zimbabweans who accuse Mugabe of trashing a promising African democracy but Pretoria has been powerless to prevent Zimbabwe's economic and political decline over the last two decades.

Grace Mugabe, 52, has asked for diplomatic immunity in the case but has not been charged. Her whereabouts were not known on Thursday although police minister Fikile Mbalula said she remained in the country.

The alleged victim, 20-year-old Gabriella Engels, has accused Mugabe of barging into a hotel room on Sunday where Engels was waiting to meet one of Mugabe's sons, and whipping her with an electric extension cable.

Mugabe had been due to appear in court on Tuesday afternoon under a deal struck with authorities but failed to turn up, police said.

Given the potential diplomatic fallout, prosecutors could decide not to pursue the case if they thought the injuries to the alleged victim were not too severe- "

On Thursday, Mbalula said a "red alert" had been issued to South African border posts to prevent Grace Mugabe fleeing.

"We had already put tabs on the borders, in relation to her leaving the country, so there's no question about that," he said. So far she had made no attempt to flee, he added.

The South African government has made no official comment on the case and foreign ministry spokesmen have not answered their phones for two days but the issue is causing waves at the highest level.

Mugabe came to South Africa for medical treatment to an injured foot, almost certainly invalidating her claims to diplomatic immunity, legal experts said.

However, given the potential diplomatic fallout, prosecutors could decide not to pursue the case if they thought the injuries to the alleged victim were not too severe, criminal attorney Riaan Louw said.

Harare has made no official comment and requests for comment from Zimbabwean government officials have gone unanswered.