Ethiopia's military strongman, Lt. Col. Mengistu Haile Mariam, today accused the Sudan of "armed intervention" in the northern part of the country and called upon all Ethiopians to prepare for a general mobilization to defend the nation's national unity.

In a nationwide speech, the Ethiopian leader charged that Sundanese troops using tanks and heavy artillery were being deployed in support of opposition forces belonging to the Ethiopian Democratic Union and the Eritrea Liberation Front and fighting in the north and northwest.

He said the Sudan was "openly violating" the country's border and that unnamed "imperialist" were coordinating the aggression against Ethiopia's territorial unity in collaboration with "neighboring reactionary Arab states."

His hard-hitting speech, which included a biting denunciation of Sudaneses President Jaafar Nimeri, appears to herald the beginning of open warfare between the two neighboring countries after months of slow deterioration in their relations stemming from the Sudan's support of the Democratic Union and Eritrean Front.

The Democratic Union is a coalition of anti-Marxist elements fighting to overthrow the Marxist military government here, while the Eritrean Front is seeking the independence of Ethiopia's northernmost province of Eritrea. Both groups operate from the Sudan and Nimeria has recently changed his policy and begun supporting them openly.

Mengistu said the situation in northern and western Ethiopia has reached a stage of "very serious concern" and that fighting was intense around several towns that were reported by Sudan to have fallen to one or the other opposition group.

He said Ethiopia had lost all patience with the aggression being shown by Sudan against Ethipia's unity and socialist revolution and indicated that the military government was preparing a major counter-offensive that would involve mobilization of large numbers of civilians.

He charged that Nimeri, acting in league with Egyptian President Anwar Sadat, had decided to dismantle Ethiopian unity and reverse its revolution "within the next three months."

Nimeri, he said, had been emboldened by his recent adherence to the Egypitian-Syrian military defense pact to step up his support for the Democratic Union and Eritrean Front.

Mengistu came as close as any Ethiopian official has so far to stating that seven towns in Eritrea and Begemdir provinces had been either lost or were in serious danger of falling to opposition forces. He said the towns were the object of "an aggression supported by foreign forces" and that the fighting to capture two of them on the Sudanese border - Metemma and Tessenei - "with the fire support of Sudanese artillery and tanks" had greatly intensified.

He promised to disclose in the next few days details of the military situation in the north and west, something the government has never done despite the intense fighting there for some months now.

Mengistu hinted strongly that the government was planning some kind of mass mobilization of civilians saying that in a short period of time "an emergency call of the motherland" would be addressed to all Ethiopians. "Wait in readiness for this proud, extremely necessary and historical sacrifice," he told the country.

The 39-year-old military leader, who presides over the ruling Provisional Military Council as its chairman, also implicated Egypt in the attacks against Ethiopia for the first time and alleged that Egyptian troops had joined the Sudanese army in a buildup along the Ethiopian-Sudanese border "to provde artillery and tank support" to the Democratic Union and the Eritrean Front.