Radio Uganda admitted today that there have been incidents involving dissidents in the Ugandan army and that some people have been killed.

The virtually unprecedented admission came on the heels of reports by Ugandan refugees here and in Tanzania that President Ide Amin has begun a purge of predoninantly Christian Langi and Acholi tribesmen in the military and police, Uganda has repeatedly denied charges by Africans and non-Africans of mass killings.

[All communications in and out of Uganda were cut for the second day Tuesday, fueling speculation that Amin might be using the blackout to cover a pogrom of Christians, David Lamb of the Los Angeles Times reported.]

The refugees claim that thousands of tribesmen have been massacred an hundreds of students from those tribes arrested in Kampala, and that Amin - a Moslem who often defends his actions by saying he received instructions from God - was replacing the Christians with Moslems and members of his own tribe, the Kakwa.

Although calls for intervention by the Organization of African Unity have increased recently, few observers - even among the Ugandan refugees - expect such as action.

In the first place, the OAU charter forbids interference in the internal affairs of a member state, and few members are willing to set a precedent that could open the door to intervention within their own borders.

Beyond this, the OAU would risk its ability to speak as the single voice of the continent and its authority ot solve disputes between members if it became a vehicle for correcting wrongs inside individual countries.

Rather, as an American-educated Ugandan here said, "Change in Uganda can only come from Arab countries" - principally Libya, Saudi Arabia and Kuwait, which have repeatedly helped bail Amin's regime out ot its chronic economic crises.

Aid from those countries has also helped Uganda pay for its Soviet made arsenal of Mig jets, surface-to-air missiles, tanks and artillery. In addition, hundreds of Ugandan soldiers and civilians are training in the Soviet Union, and some exiles express the hope that "one of them will return and overthrow Amin."

But Amin claims to be securely in power, and many exiles say that the military leaders who back him - such as his recently appointed vice president, Maj. Gen. Mustapha Andrisi - are just as bad.

Today Amin sent telegrams telling the leaders of the Central African Empire, Jean Bokassa, and of Zaire, Mobutu Sese Seko, that he is not worried by the "fake and malicious propaganda emanating from Nairobi, Dar es Salaam and Lusaka" - capitals respectively of Kenya, Tanzania and Zambia - because his military forces could meet any challenge "within one hour."

He also, according to Radio Uganda, sent messages of condolence to "the relatives of those who lost their lives during the calming down of the situation" after an uprisong within the military.

The broadcast said Amin also extended his gratitude to "members of the armed forces who, after being overpowered by dissidents in the army who were collaborating with the exiles, Zionists and imperialist agents to cause chaos in the country, were able to calm the situation."

Amin has said that a plot against him was uncovered following the discovery of a cache of arms near the home of Uganda's Anglican archbishop, Janani Luwum, who died last week in mysterious circumstances shortly after his arrest.

Amin has said that the archbishop and two Christian Cabinet ministers arrested with him were killed in an automobile accident when they tried to overpower the officer who was driving them from one place of confinement to another.

Church and other sources have charged, however, that the three were murdered, and that their deaths were part of a systematic persecution of Christians by Amin. About half of Uganda's 11 million people are Christians and about 15 per cent are Moslems.

About two weeks ago, Archbishop Luwun and 18 of his fellow bishops sent Amin a letter complaining, in mild terms, about the treatment accorded Christians in the country and about excesses by the security forces.

Meanwhile, the secretary general of the All-Africa Conference of Churches sent Amin a telegram asking him to allow the bodies of Archbishop Luwun and the two Cabinet ministers to be exhumed and examined by internation experts.

This, Canon Burgess Carr said, is "the only means of removing the blemish which their death has inflicted on Uganda."

[In London, Prime Minister James Callaghan told Parliament that Britain is calling for an investigation into the deaths by the U.N. Human Rights Commission. He was responding to a question about Amin's possible attendance at the conference of Commonwealth leaders scheduled to be held in London in June.]