An attempt by a company of dissident soldiers to seize Juba's airport in southern Sudan a month ago was part of yet another Libyan-backed plot to overthrow President Jaafar Nimeri and install a rightist government with close ties to Libya, according to Sudanese officials here.

The obscure and short-lived episode Feb. 2 was scarely reported in the Western press, and few details have been released by Khartoum, apparently because it is awaiting the completion of its investigation of the plot.

Southern Sudanese sources say, however, that the rebels' objective was to establish a base in the south from which to launch an attack on the Nimeri government in the north and destroy the accord - signed five years ago this month - that ended the 17-year-long civil war in this region.

Judging by the extensive security measures here last week for President Nimeri's two-day visit, marking the fifth anniversary of the so-called Addis Ababa agreement, the central government also regards the Juba airport incident as much more than an isolated and localized event.

It was the fifth known attempt to overthrown Nimeri in the eight years he has ruled Sudan, and the second one in the past nine months in which Libya and Ethiopia, this country's neighbors to the north and east, were allegedly involved.

Soldiers belonging to the army's air defense force here seized and held the airport for the rebels, according to Western residents here. Ten soldiers and four civilians were killed in the fighting, including an American pilot, Harold Bowman, who was killed while driving to the airport with four Swedish relief workers. Bowman, who worked for a group called the African Committee for the Rehabilitation of Southern Sudan, was reportedly on his way to fly the Swedish group on a mission during a lull in the fighting, an action deemed fool-hardy by nearly everyone here.

During the airport battle, equipment for a new America-built radio station that was to have been inaugurated during Nimeri's visit was destroyed.

In addition to the 35 arrested or captured here, others involved in the plot have been taken into custody in Wau and Malakal, two other principal towns in the southern region, suggesting that it was not just Juba the rebels were out to seize.

Gen. Joseph Lgu, commander of the Sudanese army's 1st Division, based here in Juba, described the airport attack as a "minor disturbance" by a "few short-minded people" financed and organized by enemies of the Nimeri government.

"I'm not worried because I know the quality of the people they are trying to recruit, "he said in an interview.

While hesitant to discuss in detail who was behind the plot or what their objectives were, Gen. Lagu said that "well over half" of those involved were northerners. He said they were in the pay of rightist politicians who ruled this country at various times before Nimeri's bloodless coup in May 1969.

The regional minister of culture and information, Mading deGarang, told a small group of Western and African journalists attending "Unity Day celebrations" here March 3 that the airport takeover attempt was part of the continuing effort by rightist Moslem fanatics, known as Mahdists, to overthrow Nimeri, with the help now of Libyan leader Col. Muammar Qaddafi. He also alleged that the Ethiopia military government was backing the latest plot.

July 2, and attempt by Mahdist element under the leadership of Sadiq el Mahdi - a former Sudanese prime minister and great-grandson of a 19th century Moslem religious leader - very nearly succeeded in taking control of Khartoum and toppling Nimeri. The government regards the abortive but bloody coup attempt as a "full blown invasion planned prepared and financed" by Qaddafi in connivance with Ethiopia.

Both Libya and Ethiopia have denied any involvment in the July plot, although the Sudanese have produced a mass of evidence to support their allegations.

The information minister said that Libya and Ethiopia were "very actively involved" in the latest plot, which he said provided for four planes to fly in from Ethiopia once Juba airport was under rebel control bringing Sadiq el Mahdi and Father Philip Abas, another opposition leader, and other rebels forces.

De Garang said that the northerners behind the latest plot had recruited some disgruntled southern soldiers - but no officers - by promising them higher ranks and salaries. "They were dissatisfied that they were not promoted," he said.

He said the plan called for eliminating the entire present regional leadership and dividing the country into four states, each with the regional autonomy the south alone has now. The president of the southern state - to be called the Imatong Republic after a mountain range along the Uganda border - was scheduled to be Joseph Oduhu, a top southern rebel leader during the civil war.

Oduhu, presently a member of the regional assembly, is now in jail here for plotting against the local and national governments last year. He had opposed the 1972 accord because he felt it did not give the south enough autonomy, according to De Garang.

The central government seems to have nipped the plot and maintained its total control of Juba, but whether it has weeded all the dissidents out of the army and is thus safe from Libyan-encouraged conspiracy cannot yet be answered.