Gunshots reverberated for hours last night in North Khartoum and in Omdurman, just across the Nile, and tanks stopped traffic at checkpoints on two of this city's main bridges this morning after a pair of apparently spontaneous mutinies among discontented soldiers.

[The state-run radio later quoted the defense minister, Gen. Osman Abdullah, as saying the rebellion by "a small group in the Sudanese armed forces" had been crushed after the death of one soldier and injuries to seven, United Press International reported. Abdullah said the mutineers were "motivated by ethnic ends."]

In violence Saturday, also involving soldiers, gangs of Arab Moslem northern Sudanese were fighting black African Christian Sudanese from the south. At least four persons were killed.

Last night, according to diplomatic sources, the violent incident in the medical corps based at Omdurman appears to have been the action of northern troops unwilling to obey orders sending them to the war zone in the south. But according to these same sources, the mutiny in North Khartoum was the work of rebellious southerners in the ranks.

Diplomatic and Sudanese political sources suggested the roadblocks this morning were meant to apprehend deserters fleeing after the mutinies were suppressed.

Nine days ago, angry marchers burned a flag outside the Embassy of Egypt, Sudan's powerful northern neighbor, and shouted anti-American and anti-Egyptian slogans, raising sharp protests from Cairo.

Yesterday, for the second time in two weeks, striking radio operators shut down the Khartoum airport, forcing the cancellation of all commercial flights for 24 hours.

Street violence and mass protests led to the overthrow of Jafaar Nimeri nearly six months ago and there are rising concerns that his successors, a disunited military-civilian coalition, may soon fall to a similar fate.

"Things could continue at this level for months or it could become very serious very quickly," said a senior western diplomat.

The U.S. Embassy and other diplomatic missions are attempting to ride out the storm with as low a profile as possible. But Egypt sees this country that controls the upper reaches of the Nile River as vitally important to its own interests.

Washington made Sudan under Nimeri a cornerstone of regional policy and currently provides hundreds of millions of dollars in military and development assistance.

Sudan borders nine nations and the Red Sea and if it were to align with Soviet-backed Ethiopia, Washington could face a serious geopolitical challenge in the Horn of Africa.

The variety of untested groups now contending for power after Nimeri's 16-year rule makes the outcome of any confrontation unpredictable.