Get breaking news alerts and special reports. The news and stories that matter, delivered weekday mornings.

A deadly outbreak of anti-foreigner violence has forced about 10,000 immigrants to flee their homes in South Africa, the head of a leading humanitarian organization told NBC News on Thursday.

At least four people have been killed in two weeks of xenophobic violence in the port city of Durban — and there are fears the unrest will spread.

“Over the last 48, 72 hours, it has escalated substantially,” Imtiaz Sooliman, the founder and director of Gift of the Givers, which has helped set up five temporary camps for fleeing immigrants in and around Durban. "You are now talking about 10,000 people who have been displaced.”

“These camps were basically set up very quickly ... the violence is spreading,” said Sooliman, whose organization is one of the largest NGOs on the continent.

The Morning Rundown Get a head start on the morning's top stories. This site is protected by recaptcha

President Jacob Zuma condemned the attacks and urged for calm on Thursday.

Meanwhile, many immigrants in and around the commercial capital of Johannesburg have shuttered their shops. Xenophobic attacks in Johannesburg in 2008 killed more than 60 people.

"Foreigners in Johannesburg have been observed closing their shops. We don't know why but police are keeping an eye on the situation," Johannesburg police spokesman Wayne Minnaar said, according to Reuters.

A South African police officer looks on as immigrants clear items from their shop for fear of attacks in Primrose outside Johannesburg on Thursday. SIPHIWE SIBEKO / Reuters

Reuters reported that a text message sent to many foreign shop owners warned that "Zulu people are coming to town ... to kill every foreigner on the road.”

Around 5 million immigrants are thought to live in South Africa, which has a total population of about 50 million. At least one-in-four people are believed to be jobless, which along with the widespread poverty is thought to feed violent anti-immigrant feelings.

Tensions were running high Thursday in Diepsloot, a township north of Johannesburg that has often been the site of anti-immigrant attacks, a local journalist told NBC News.

"It is actually getting worse you see in the township of Diepsloot — you are seeing 'mob justice' and that mob justice is something that usually happens to foreigners, " according to the journalist, Golden Mtika.

Many of the locals were angry and resentful that immigrants from neighboring countries as well as Bangladesh and Pakistan were setting up businesses and thought to be taking away jobs from native South Africans, he added.

Reuters contributed to this report.