UPDATED 3.13PM: South African soldiers have been deployed to tackle gangs hunting down and killing foreigners after at least seven people died in a wave of anti-immigrant violence.

AFP photographers witnessed dozens of military police surrounded a downtown Johannesburg hostel housing mainly South Africans which police raided as a helicopter hovered above.

Inside the hostel police moved from one floor to another conducting searches as residents lay face down in corridors.

Around the neighbourhood, soldiers stood in groups of around six, positioned a few metres from each other.

In recent days police in the economic capital Johannesburg and in the port city of Durban have struggled to contain mobs targeting migrants from Zimbabwe, Malawi, Mozambique and other African countries over the past three weeks.

The government has vowed to crack down on the unrest, but the decision to put soldiers on the streets came after two nights of relative quiet in both cities.

"We come in as the last resort - the army will serve as a deterrent," Defence Minister Nosiviwe Mapisa-Nqakula told reporters earlier Tuesday.

"Now we are deploying because there is an emergency."

The spate of attacks has revived memories of xenophobic bloodshed in 2008, when 62 people were killed in Johannesburg's townships, tarnishing South Africa's post-apartheid image as a "rainbow nation" of diverse groups living in harmony.

In Johannesburg, the military presence will focus on flashpoints such as downtown and the township of Alexandra, a poor neighbourhood recently roiled by xenophobic clashes, including the stabbing death of a Mozambican man in broad daylight on Saturday.

Graphic photographs of the killing were published by many South African and international newspapers and websites.