President Jaafar Nimeri, who brought peace to Sudan by ending a debilitating 17-year civil war in the south of this country, today faces a new upsurge in violence there that threatens to undermine what is still widely regarded as his major accomplishment.

The deteriorating security situation in the southern part of the country and the renewed high level of tension it has created between the African-inhabited south and the Arab north appear to be the major preoccupations of the Nimeri government--far more than the threat, sometimes real and sometimes imaginary, of an invasion from Libya.

Nimeri's government has supported Egypt's participation in the Camp David peace process and the expanding American military presence in the Middle East. The Reagan administration has made clear that it views both internal and external threats to Nimeri's survival with alarm.

No one is talking yet about a resumption of the civil war that ended with a peace accord signed in Addis Ababa 11 years ago. But large bands of men armed with AK47 automatic rifles and calling themselves Anyanya Two--after the earlier separatist movement--are roaming the south again, attacking police stations, recruiting men and killing merchants from the north.

The accord, signed in the Ethiopian capital in March 1972, ended a war in which the south's African and Christian-led population had fought for independence from the Arab and Moslem north. Several hundred thousand persons died in the struggle, and more than a million southerners fled to neighboring countries.

The agreement united the three former southern provinces into a single autonomous region with an elected regional assembly and executive committee.

But one diplomat here said, "There is a new spirit in the south and a feeling the Addis Ababa agreement has not brought them anything. People are going back to the bush."

In mid-January, one band, reportedly numbering 200 men, rounded up northern merchants at a railroad station north of Aweil and killed 14 of them, either by cutting their throats or riddling them with bullets. It was the worst, though not the first, incident of its kind and set off alarm bells in Khartoum.

The government acknowledges a serious breakdown of law and order in parts of the south but says it is the work of "bandits" and "outlaws" who are not politically motivated. It has responded by sending more troops to back up the poorly armed police force.

But outside analysts and southern politicians note the involvement of several dissident figures, one a cashiered Army colonel with Ethiopian and Libyan backing, and two former Anyanya members. They say there is strong evidence that the violence is at least partly political in motivation and aimed either at destabilizing Nimeri's government, already hard-pressed on the economic front, or protesting government policy toward the south.

The upsurge in lawlessness comes against a backdrop of spreading unrest over a two-year-old Nimeri proposal, supported by one southern faction, to end the special autonomous status of the south and redivide it into three regions as part of a nationwide plan of decentralization.

The issue has paralyzed the south's regional assembly. A large majority of its 116 deputies, led by the dominant Dinka tribe, is vehemently opposed to division while a highly vocal minority based in the Equatoria region is just as strongly in favor of it.

After a storm of protest, Nimeri canceled plans to hold a referendum on the issue last year. But he is still being pressed by Vice President Joseph Lagu, who is from Equatoria, to push through the division.

Caught in the middle of the political turmoil and increasing violence is the American oil company, Chevron, which has discovered oil in the south and is about to begin building a $900 million pipeline north to the Red Sea. Oil royalties could provide this virtually bankrupt nation with its first major source of income.

After several incidents suggesting the pipeline may become a target of the southern dissidents, the Army last month dispatched reinforcements to its garrison in Bentiu, near Chevron's main base, to beef up protection for the pipeline project.

Chevron, which had long hoped to avoid such a measure, has now conceded the need for it after several employes were held up by a band and local tribesmen made hostile comments to others.

The general view in Khartoum among diplomats, southerners and even northern opposition leaders is that the turmoil in the south is still containable and can be dealt with politically if Nimeri takes measures now to defuse the division issue.

Nimeri, 53, is a proven master of his nation's intrigue-laden politics and during 14 years in power has survived a dozen coup attempts through courage, toughness and an acute sense of the prevailing political winds. But there are fears in some diplomatic and Sudanese circles that the president is increasingly isolated from his people and may no longer realize the depth of opposition to his division plan.

Why Nimeri decided to risk upsetting the peace in the south, already badly strained by its disastrous economic situation, internal divisions and tensions with Khartoum, by pressing ahead with the division plan is not clear.

But some of his old supporters, among them Mohammed Omer Beshir, a Khartoum university professor who served as his point man in arranging the Addis Ababa peace agreement, fear he is making a grave mistake.

"Nimeri is undermining his greatest achievement," Beshir stressed in an interview. "The last 10 years we have had peace in the south. The greatest thing he did was the Addis Ababa agreement."

One apparent reason for Nimeri persisting in his division plan is the intense pressure from his vice president, Lagu, 51, the former military leader of the Anyanya.

In an interview, Lagu, from the Latuka tribe, gave vent to what was clearly a deep bitterness toward the Dinka tribe, whom he accused of upsetting the peace by trying to impose its hegemony over the smaller tribes, such as his own, in the south.

"In the past, we opposed northern domination. Now do we have to have Dinkas replacing northerners? Nobody will accept this. Better we divide the south up into the former three provinces," he said.

The former Anyanya leader branded his Dinka opponents "peacetime heroes" and said they were suffering from "some kind of inferiority complex" because they did not join the Anyanya movement but lived comfortably in Khartoum while he was eating "wild fruits in the southern bushes."

He strongly defended the division plan, saying it was in keeping with the recent decentralization of the north into six regions and would end the problem of the centralization of power in the south in Dinka hands.

He denied that such a division would undermine the Addis Ababa agreement, arguing that the war had been fought to establish the rule of the south by southerners and that this had been fully achieved. "It is making the aspirations of the Addis Ababa agreement more realistic," he said.

The "decentralization" of the south would not bring northern domination back, he said, but would assure that "no portion of the southern population imposes its own culture and values on the other tribes or tries to rule the others."

Lagu said he intended to keep pressing for the redivision of the south and would offer as a "compromise" that Equatoria become a separate region on its own, even if the other areas of the south wished to remain united.

Whether Nimeri will accept this ploy to get around the clear majority in the southern regional assembly (83 of the 116 deputies) favoring a united south remains to be seen. Already, he is at loggerheads with the Dinkas because he has tried during the past year to break up the Dinka-led pro-unity faction by arresting some of its members, campaigning for his plan in the south and ordering new elections in the regional assembly in hopes of getting a majority favoring division.

While the security problem in the south had already set in before the division issue came to the fore, it has clearly taken on new proportions during the past year, spreading from the eastern Ethiopian border areas in the Upper Nile and Junglei provinces to Bahr el Ghazal Province in the west.

According to a well-placed Dinka source who asked not to be identified, about 1,000 dissidents entered Bahr el Ghazal by early January, mainly on a recruiting mission. The dissidents, heavily armed and carrying radio communication equipment, broke up into four groups, one of which was involved in the killing of the 14 northern merchants at the Ariat railway station north of Aweil on Jan. 18.

He and several other sources said the size of the bands roaming the south varied between 100 and 200 men each. But there seems to be no agreement on whether there is really a new "Anyanya movement" with the aim of restarting the civil war.

Lagu insisted that there is no Anyanya Two and described the bands as "a bunch of outlaws" and "a confused group of people whose objective is not known." Others say, however, that there is at least the rudiments of a new movement being put in place but that there is considerable fragmentation and deep suspicion among the various opposition groups.

The dissident groups are said to be recruiting members among various disaffected southern elements. These include former Anyanya guerrillas who failed to be absorbed into the national Army after 1972, participants in several mutinies of southern soldiers in the mid-1970s, laid-off civil servants, vagrants forcefully removed from Khartoum during the past several years and members of gangs that have sprung up as a result of the south's disastrous economic situation.

Arms are said to be plentiful throughout the south, particularly since ex-Army supporters of the ousted Idi Amin in Uganda crossed in large numbers into southern Sudan after his fall in 1979. But the dissidents are also said to be getting weapons from Ethiopia, which has traditionally used the threat of arming the southern Sudanese as leverage over the central government to get it to crack down on the Ethiopian opposition operating out of the Sudan.

The government has responded to the latest killings of northern merchants by trying to rotate southern troops to the north, a process that has not taken place for six to seven years and led to a bloody mutiny in Akobo in March 1975.

Reports reaching the capital say southern soldiers of the First Division stationed in Wau, Bor, Aweil and Bentiu have been refusing to go north and that in Bor they have barricaded themselves in the garrison and threatened to open fire if their northern replacements enter the town.

It was unclear why the Army decided to rotate the troops at this juncture. Nor was it known in Khartoum whether any of the southern soldiers resisting rotation were in contact with any of the southern opposition groups. Also unknown was how Nimeri intends to deal with the insubordination.