At the international airport on the outskirts of this Nile River capital. Soviet-made anti-aircraft guns implanted along the runway point skyward in readiness.

At night, army and other security units carry out searches across the river in Omdurman and other suburbs. It is rumored that several trucks loaded with arms were seized only last week on roads leading into the city.

Without any explanation, the government clamped a curfew on the oasis town of El Atrun last Thursday in the northwestern corner of the country near Libya and banned all traffic through the district.

Meanwhile, rumors are circulating that some Ansars, the fanatic, power-hungry followers of the Puritan Moslem Hahdi sect, are again infiltrating across the borders from Libya and Ethiopia.

"Everybody is expecting somehting to happen, but nobody knows quite what," said a Western diplomat.

Nearly seven months after the bloody, Libyan-sponsored coup attempt against Jaafar Nimeri, Khartoum is again jittery. The government's clearly suspicious that Libyan leader Col. Muammar Qaddafi, with Soviet backing and Ethiopian conni-vance is plotting more trouble.

Nimeri, the stocky, 47-year-old military Sudanese strongman, has not only become deeply entangled in big power and Arab politics. He also has to deal now with a sharp deterioration in relations with neighboring Ethiopia that threatens to drag him into the equally complicated politics of the Horn of Africa.

Fighting in two separate civil wars under the way in north and northwest Ethiopia along the Sudanese border has suddenly escalated. A direct confrontation is now threatened between the two nations, which previously enjoyed correct, if not exactly warm, relations. Both countries have already recalled their ambassadors, and Sudan Airways has suspended its flights to Addis Ababa.

Earlier this month, Nimeri warned that he might close Sudan's borders with Ethiopia and threatened publicly to use the 140,000 Ethiopian refugees now resideing in his country "to export unrest and problems" to Ethiopia if its military rulers did not crease aiding his enemies.

The Nimeri government is apparently doubly nervous these days because the second congress of the Sudan Socialist Union, the country's only political party, begins today. It is expected to nominate Nimeri as the sole candidate in a presidential referendum to be held this year and also to approve a whopping $5.6 billion six-year development plan.

Some 2,000 delegates have gathered in the capital for the occassion, which represents, more than anything else, another step by Nimeri to institutionalize his government and his ambitious economic policy.

With strong Arab and Western backing, Sudan aims to become the breadbasket of the entire Arab world in the next 25 years. Implementation of the plan, involving billions of Arab petrodollars, is almost certain to entrench the Nimeri government for years. It enemies seem only too well aware of that likelihood.

"Whenever we get ready to make a new thrust forward, thee is a strong reaction from the opposition," commented Khartoum University prof. Mohammed Omar Bechir. "It happened before, when the Addis Ababa agreement was about to be signed, and when we adopted a new constitution."

The agreement struck between the Nimeri government and Rebel leaders in southern Sudan ended a 17-year civil war that was tearing this nation apart. Signed in the Ethiopian capital in March 1972, it is widely regarded as Nimeri's single most important achievement to date.

The general assessement now seems to be that the coup attempted last July 2, in which 400 military and villians were killed in house-to-houes fighting, was a traumatic experience that has left a lasting impression on Nimeri.

About 2,000 commandos, most of them Ansars, came by convoy from camps in Libya and Laid seige to the capital just as Nimeri was returning from a trip to the United States and France. Had it not been for his early arrival in Khartoum, Nimeri might have been killed upon descending from his presidential plane.

The leader of the coup attempt. Sadiq Mahdi, he said publicly that he intends to continue trying to topple the general. Sadiq, once Sudan's premier, is the great-grandson of the 19th century Moselm leader who claimed to be the Mahdi, or savlor, and who first unified this huge nation.

Sudanese officials are also convinced that Qaddafi is acting in league with Ethiopia's radical Marxist military leaders to encourage Sadiq in his plotting.

The emerging alliance between Libya and Ethiopia forms the backdrop for the current deterioration in relations between Khartoum and Addis Ababa. The Sudanese say Libya convinced Ethiopia to help train some of the 5,500 Ansar refugees living since an abortive coup in 1970 in camps located in a remote area of western Ethiopia.

Ethiopia has denied any involvement in the july coup attempt, but the Sudanese are not convinced.

Sudan has stepped up its support for Ethiopian groups opposed to the military government, the Eritrean Liberation Front and the Ethiopian Democratic Union. The former seeks the independence of Ethiopia's Red Sea province of Eritrea, and the latter the overthrow of the present Ethiopian military government.

Despite all his foreign policy troubles. Nimeri is seen by most Western diplomats here to be gaining steadily in strength. There have been three major coup attempts, in which the Mahdists have had a hand, and a couple of minor ones since he came to power in a military coup in May 1969.

Nimeri has ended the divisive, multi-party politics this country had known since its independence in 1956, stopped the civil war in the south, established a singe-party system, adopted, and then dropped, a Socialist policy, and is now trying to pull his sparsely populated country of 17 million people out of grinding poverty.

His person and politics have, inevitably, triggered a strong reaction from those who led and profited from the old system of multi-party politicking. The Ansars, for example, had their own Umma Party that ruled the country off and on for the 13 years before Nimeri came to power.

In addition to their intense personal hatred of Nimeri, who was responsible for the killing of several thousand Mahdists on the Nile River island of Aba Aba in 1970, the Ansars oppose his strongly pro-Egyptian policy. This helps to explain Qaddafi's interest in them, for he is also dedicated to overthrowing President Anwar Sadat.

Nobody knows exactly how many Ansars, or Mahdists, there are in Sudan, but one member of the sect believes they form a quarter of the population.

The U.N. High Commission for Refugees was providing assistance to the Ansar refugees in Ethiopia, but after Nimeri's latest allegations against them, it abruptly ceased this. Nor has it been allowed to visit the camps for more than a year, raising doubts within the Commission as to what precisely is going on.