Clashes between armed rival groups in South Sudan are gradually turning into an all-out ethnic war between the two main tribes, Dinka and Nuer, as reports of mass killings based on ethnicity begin to emerge amid the ongoing bloody violence.

As feared earlier, government forces backing President Salva Kiir, who belongs to the Dinka tribe, shot dead dozens of people on the outskirts of the capital Juba, mostly Nuer neighbourhoods, according to local reports.

The allegation by opposition groups has been denied by the army.

Simon, who gave only his first name, told AFP that he had witnessed Kiir's troops rounding up more than 200 Nuer tribals and shooting them dead. He is presently sheltered at a UN building in Juba.

"It was horrible, because to survive you had to cover yourself with the bodies of dead people, and... the bodies started to smell really bad. I don't want to talk much about it," said Simon.

Other witnesses at the same UN base have echoed similar versions of the alleged ethnic massacre. There are about 45,000 civilians in the shelter at the base in Juba.

South Sudan has been reeling under intensified violence in the past few days as rival army factions launch attacks against each other. Officials have been expressing concern that the situation could easily spill over into a bloody ethnic conflict.

Kiir has been accusing dismissed opposition leader Riek Machar, who hails from the Nuer tribe, of attempting a coup against the government leading to the clashes, which are believed to have claimed at least 1,000 lives so far.

Meanwhile, the UN chief has called for a sharp increase in the international troop presence in order to quell the spiralling violence.

Secretary General Ban Ki-moon, in a letter to the 15-member Security Council, has called for 5,500 more peacekeepers in South Sudan. The landlocked middle African country currently has about 6,700 UN troops and 400 police personnel.