KIGALI, April 7 (Reuter) - The presidents of Burundi and neighbouring Rwanda were killed on Wednesday night when a rocket downed their plane as it landed in Rwanda, Western diplomats and the United Nations said.

The two presidents, whose countries have been racked by tribal violence, were returning from a regional summit of African leaders in the Tanzanian capital Dar Es Salaam.

Rwanda's U.N. ambassador told Security Council members in New York that the attack was an assassination.

Diplomats in the Burundian capital of Bujumbura said they were convinced both Burundi President Cyprien Ntaryamira and Rwandan President Juvenal Habyarimana had died in the attack.

U.N. Under-Secretary-General Chinmaya Gharekhan said in New York he had been informed by the U.N. special representative in Rwanda that "both presidents lost their lives".

Gharekhan said Rwanda's U.N. ambassador told members of the Security Council that "it was not an accident. It was an assassination. There was rocket fire at the plane...The plane was brought down as it was landing and the presidents were killed".

Gharekhan said the United Nations had no independent confirmation that assassination was involved.

The United Nations has a 2,500-member peacekeeping force in Rwanda, which is still recovering from a three-year civil war between the majority Hutu and the Tutsi tribes.

Security Council President Colin Keating of New Zealand told reporters: "The tragedy is all the more acute because of the grave situation facing those two countries.

"Therefore the members of the Security Council have asked me to reaffirm the council's desire that calm prevail in the next few days."

Keating said the United Nations would have to review the situation of U.N. personnel immediately "to ensure their safety and that they are being deployed to the best effect".

At the meeting both presidents attended in Dar Es Salaam, Tanzania, Kenya and Uganda agreed to send foreign ministers to Burundi to help rebuild confidence in the government.

The talks were aimed at finding ways to end Hutu-Tutsi tribal violence in Burundi and Rwanda.

No mention was made in a statement at the end of the summit of Tanzanian proposals for sending a peacekeeping force.

The leaders also called for reforms to the Burundian army, dominated by the minority Tutsi and seen as largely responsible for clashes since October with the Hutu majority.

Ntaryamira said he was grateful for neighbouring countries sheltering 800,000 Burundi refugees from the tribal fighting since renegade troops killed Burundi's first Hutu president, Melchior Ndadaye, on October 21 last year in a failed coup.

The United Nations says 375,000 Burundians are registered as refugees in Zaire, Rwanda and Tanzania.

"Burundi is bleeding. I am aware it hurts your economies. Still, we need your help." said Ntaryamira.

On Rwanda, the African leaders said they were concerned at delays since December in forming transitional institutions as agreed in the northern Tanzanian town of Arusha last August.

The U.N. Security Council on Tuesday renewed the mandate for peacekeeping forces for Rwanda for four months but threatened to pull them out unless the Arusha peace agreements were honoured.

(c) Reuters Limited 1994