Somalia yesterday accused the Ethiopian military government of preparing to launch a "war of aggression" against it with the help of Cuban forces still stationed in the Ogaden region of southeastern Ethiopia.

At a press conference here, Somali Ambassador Michael Mariano warned Africa and the West that "very soon" Somali dissidents would begin attacking Somalia under the guise of a "war of liberation."

He charged the whole strategy had been worked out earlier this month in Addis Ababa during the Afro-Arab solidarity conference attended by Cuban President Fidel Castro and Soviet First Vice President Vasili Kiznetsov while Ethiopia was celebrating the fourth anniversary of late Emperor Haile Selassie's overthrow.

(In London, the Ethiopian Embassy categorically denied the Somali charges in a statement asserting that they were "calculated to camouflage" Somali preparations "for another full scale invasion of Ethiopia," Reuter reported.)

With Somali insurgents reportedly still operating inside the Ogaden, the possibility of still another border war between Somalia and Ethiopia remains considerable. Somalia has already charged Ethiopia with bombing several towns inside its territorysince March and the Ethiopians have repeatedly warned they would not sit by idly if Somali irregulars continue to harass Cuban and Ethiopian troops stationed in the Ogaden.

Mariano said his government had just received confidential information regarding the new Ethiopian-Cuban-Soviet strategy which he described as aimed simultaneously at crippling the Somali Army and wiping out Somali insurgents still "genuinely struggling for their freedom" inside the Ogaden.

At another point, however, he said the main objective of three countries was "the domination of Somalia."

He estimated that between 30,000 and 50,000 Ethiopian and Cuban troops would be involved in the operation together with "so-called (Somali) liberation forces" being used to spearhead the attack, expected "by the end of October."

Mariano insisted that his government's information on the Ethiopian-Cuban-Soviet plan was well founded and accurate but refused to disclose the sources who has provided it. He noted that Somali warnings last year about the Cuban buildup in Ethiopia - at first dismissed in the West as vastly exaggerated - had finnaly proven far more accurate than those of Western intelligence agencies.

Western observers here noted that Ethiopia had made similar allegations against Somalia prior to the war between the two countries over the Ogaden in mid-1977 as Somalia insurgents began infiltrating the disputed region.

Thus there was speculation here that Ethiopia might indeed be planning to take its revenge on Somalia now by sending dissidents back into Somalia to undermine the government of President Mohammed Siad Barre.

The Soviet Union and Cuba might also have an interest in such a plan following Siad Barre's decision last November to sever diplomatic relations with Cuba and oust all Soviet military personnel from Somalia. His aition was taken in retaliation for Soviet and Cuban assistance to Ethiopia.

The disputed Ogaden is a vast tract of semi-desert land along the Somali-Ethiopian border that is sparsely populated by Somali-speaking nomads. The Somali government had long laid claim to the territory as part of a "greater Somalia" incorporating all Somali peoples in the region into one nation.

The two countries have now fought two wars over the Ogaden both of which Ethiopia has won, leaving the Somalis embittered and frustrated each time.

Somali insurgents succeeded in occupying virtually the entire Ogaden proper plus considerable additional Ethiopian territory to the west and north before the Ethiopians drove them out last March with massive Cuban and Soviet military assistance. The Soviet Union rushed an estimated $1 billion worth of arms to Ethiopia and Cuba sent 17,000 combat troops to help the Ethiopian Army recapture the Ogaden.

Mariano charged yesterday that Ethiopia had been recruiting "Somali outlaws" who fled the country, including some 20 officers and soldiers involved in a recent coup attempt against Siad Barre's government. Many more whose families live in the Ogaden have been forcefully recruited while their families are held as hostages," he said.