More than 17,000 people are estimated to have fled attacks in the past two weeks, including refugees and asylum seekers who went to South Africa seeking protection from persecution in their homelands, UNHCR spokeswoman Jennifer Pagonis said on Friday.

Mobs accuse African migrants of taking jobs and fuelling crime. The attacks spread from Johannesburg to Cape Town on Friday, where migrants' homes and shops were looted.

A very large percentage of those displaced by the xenophobic violence are Zimbabweans, Pagonis said, including people who had come to South Africa to seek asylum. "They urgently need both assistance and protection," she told a news briefing in Geneva.

Although most of the Zimbabwean migrants are in South Africa illegally, it is not feasible for them to return home to escape the violence.

"While thousands of Mozambicans are reportedly streaming home, many Zimbabweans cannot consider returning home due to the well-known situation in their country," she said, referring to post-election violence and other problems in Zimbabwe.

The UNHCR was "urging South Africa to exceptionally grant to Zimbabweans the possibility to regularise their stay in the country," an option foreseen under its national law, she said.

"Recent events in South Africa, as well as in their own country, are once again highlighting just how vulnerable this group is, making acting on UNHCR's appeal even more urgent today," Pagonis said.

UNHCR spokesman Andrej Mahecic clarified that this amounted to a request to allow the Zimbabweans to remain in South Africa temporarily.

Under South African regulations, foreigners who overstay their visas or enter illegally can be deported. Mahecic said South African laws made it possible to grant temporary residence permits in special circumstances.

UNHCR has distributed blankets and mats to the displaced staying at sites near police stations in Johannesburg.

There are more than 125,000 registered refugees and asylum seekers in South Africa from a wide range of countries, it says.

Migrant workers and other foreigners have approached the Pretoria regional office of the International Organisation for Migration (IOM) seeking help to leave, a spokesman said.

"But most of them have no money. They fled literally with the clothes they had on their back and most of them have no documentation because of their status," IOM spokesman Philippe Chauzy told reporters in Geneva.

Mozambique was trying to evacuate about 10,000 of its nationals from South Africa and buses were lined up, he said.