JOHANNESBURG (Reuters) - Presidents Nelson Mandela of South Africa and Robert Mugabe of Zimbabwe met privately on Friday to discuss rising tensions between Angola and Zambia over Angola`s civil war, diplomats and government sources said. The two heads of state were also to talk about the widening war in the Democratic Republic of the Congo, but Congo President Laurent Kabila is not expected to attend the meeting. Mugabe, who flew to South Africa secretly on Thursday night, was meeting Mandela at an undisclosed venue to discuss worsening ties between Zambia and Angola over Luanda`s allegations that Lusaka is backing the rebel UNITA movement, the sources said. Tensions have run high between Zambia and Angola since December when dos Santos` government accused Zambia, Rwanda, Uganda, Togo and Burkino Faso of providing logistical military support to UNITA led by veteran guerrilla fighter Jonas Savimbi. UNITA, the National Union for the Total Independence of Angola, and the MPLA government in Luanda resumed fighting in December after the collapse of a 1994 peace accord. Zambia, which mediated the peace deal, denies the charge and says the allegations stem from its refusal to allow Angolan forces to attack UNITA from Zambian soil. A spate of bombings rocked Lusaka last weekend, including an explosion inside the Angolan embassy which killed an Angolan security official and wounded another man. Mandela is hosting the meeting in his role as chairman of the Southern African Development Community.