* Pretoria issues "stern warning" against "illegal activities"
* Rwanda denies involvement, asks to be shown proof
* Row strains ties between players in east Congo
* Focus on dissidents' killings is embarrassment for Kagame (Adds Rwandan foreign minister's comment, military analyst)
By Wendell Roelf
CAPE TOWN, March 12 (Reuters) - South Africa delivered a diplomatic slap to Rwanda on Wednesday, warning it would not tolerate "criminal" attacks on its soil against Rwandan exiles that have drawn international criticism of President Paul Kagame's government.
Issuing a "stern warning" against such activities, Justice Minister Jeff Radebe said Pretoria had evidence linking recent attacks by gunmen against exiled critics of Kagame to three Rwandan diplomats and one Burundian envoy who were expelled last week. But he offered no details of this evidence.
In retaliation, Rwanda expelled six South African diplomats on Friday, straining ties between two African states which have been involved, in differing ways, in recent conflict in eastern Democratic Republic of Congo. South Africa has troops in a U.N. brigade that fought last year against Congo rebels whom U.N. experts said were backed by Rwanda. Kigali has denied this.
Rwanda's ambassador to South Africa said Pretoria had shown no proof of Kigali's involvement in the attacks against exiles. Kagame's government has accused South Africa of sheltering "dissidents responsible for terrorist acts in Rwanda".
"That is the issue Rwanda has with the South African government. As for expelling Rwandan diplomats, it was simply wrong, and Rwanda has every right to reciprocate," Rwandan Foreign Minister Louise Mushikiwabo said in a statement on her Twitter account.
Radebe told reporters in Cape Town the two countries were maintaining diplomatic relations. But he said: "As the South African government, we want to send a very stern warning to anybody, anywhere in the world, that our country will not be used as a springboard to do illegal activities".
The row followed a raid by armed men early last week against the Johannesburg home of former Rwandan army chief General Faustin Kayumba Nyamwasa, an exiled critic of Kagame, who also survived an assassination attempt in South Africa in 2010. He was not at home at the time of last week's attack.
South African police have also been investigating the New Year's Eve murder in a posh Johannesburg hotel of another exiled Kagame opponent, former Rwandan spy chief Patrick Karegeya.
Exiled Rwandan opposition members have accused Kagame and his government of being responsible for Karegeya's death and for the attacks on Nyamwasa and other overseas-based critics.
Kagame, who has won Western praise for rebuilding Rwanda after the 1994 genocide, denies his government ordered the attacks, but has said "traitors" should expect consequences, a remark that dismayed Western donors of the Great Lakes state.
The United States has expressed concern at what it calls "politically motivated murders of prominent Rwandan exiles".
The row with South Africa is an embarrassment for Kagame as he prepares to commemorate the 20th anniversary of Rwanda's genocide next month.
"KAGAME PUSHES HIS LUCK"
Rwanda's 1994 genocide saw Hutu soldiers and militia slaughter around 800,000 mostly ethnic Tutsis, while the international community largely stood by.
Critics say Kagame, who led his predominantly Tutsi rebel movement to power after the genocide and won support from Western powers as an ally in turbulent central Africa, has taken advantage of Western guilt over the genocide to increase persecution of opponents.
Last year, he faced criticism over U.N. experts' reports showing his government supported an insurgency in eastern Congo by Tutsi-led rebels of the M23 movement. Kagame has denied this, but the United States blocked military aid.
South African soldiers were among U.N. peacekeepers who helped subdue the M23 revolt last year.
"Mr Kagame has fought his way out of tougher corners than this damaging row with SA (South Africa). But sending hitmen abroad, if that is what he did, smacks of desperation," South Africa's leading business daily, Business Day, said in an editorial this week entitled "Kagame pushes his luck".
South African military analyst Helmoed Romer Heitman told Reuters President Jacob Zuma's government was likely to be wary of escalating the dispute with Rwanda, which despite its small size punches well above its weight in regional affairs.
"It's completely in Kagame's power to cause a lot of trouble in the DRC if he chooses to. So you don't want to irritate him too much; on the other hand we can't have people running around killing people in our country," Heitman said.
Rwandan High Commissioner to Pretoria, Vincent Karega, speaking from Kigali, said Rwanda had been presented with no evidence linking its personnel to the attacks on dissidents. "As far as we are concerned we haven't seen any proof," he said.
"We have evidence," Radebe said, adding the expelled diplomats had violated their diplomatic status by being associated with "illegal activities that have taken place where there were attempted murders, including a murder".
After meeting Kagame this week, the U.S. Special Envoy to the Great Lakes region, Russ Feingold, said Washington was very worried about the South Africa-Rwanda dispute, "because they are both critical countries to the future of Africa". (Additional reporting by Ed Cropley and Pascal Fletcher in Johannesburg and Jenny Clover in Kigali; Writing by Pascal Fletcher; Editing by Giles Elgood)