Early last month in this mountainous land strategically located at Saudi Arabia's southern back door, two of the government's fighter bombers were shot down by local guerrillas. Both the government's planes, Sukhoi 22s, and the guerrillas' antiaircraft weapons, were supplied by Moscow.

The incident illustrates the extent to which the Soviet Union has deftly positioned itself on all sides of the newly escalating guerrilla war here--a conflict in which the United States is conspicuous by its relative absence.

Three years ago, the Carter administration sent $390 million in arms here to demonstrate U.S. determination to "draw the line" against Soviet expansionism in the Middle East. But President Carter for a variety of reasons appeared to lose interest in the conflict, and the Reagan administration apparently has decided to ignore it despite its implications for the long-term security of Saudi Arabia, the United States' principal foreign supplier of oil.

Officials in the government of North Yemen President Ali Abdullah Saleh say it was this U.S. attitude, coupled with endless delays in Saudi aid, that forced them in the face of an insurrection to turn to the Soviets. But while Moscow was glad to oblige with between $1 billion and $2 billion in weapons and about 600 military advisers in recent years, according to Western diplomatic sources, it has continued to arm the guerrillas indirectly through Marxist South Yemen.

"If things continue as they are, North Yemen will be a Communist state within five years," warned one well-informed Yemeni official.

A Western diplomat was less alarmed about North Yemen's immediate fate, but added "the parallels with Afghanistan are there . . . . But I don't think it a Communist takeover will happen at least for another generation."

North Yemen serves as a natural land corridor into the Saudi kingdom and the uncontrolled flow of people, goods and arms between the two nations long has been a problem.

So has Saudi influence here, which many Yemenis view as overweening and oppressive. The kingdom is North Yemen's main financial backer. At the same time, however, the Saudis give millions of dollars in bribes to dissident northern tribes, thus assuring a Saudi zone of influence.

Many experts believe the Saudis have always been extremely ambivalent about Yemen. Although economically weak, the two Yemens combined have a population of 9 million, nearly double Saudi Arabia's 5 million.

"The Saudis have never been able to decide whether they want a strong Yemen or a weak Yemen," said one Yemeni analyst. "But Yemenis tend to blame them for keeping the country divided and weak."

Like every subject under discussion here, there is no agreement among diplomats and Yemeni analysts about what Soviet strategy is toward North Yemen, except that it is long-term and persistent. But there can be little doubt about Moscow's growing presence and importance here.

Three years ago, Carter administration officials estimated there were 200 or fewer Soviet advisers here and noted they were mostly confined to an air base outside the coastal port of Hodeida, where they trained Yemenis on Mig aircraft.

Today, Western diplomatic sources concur that the number of Soviet advisers have roughly tripled and say that in addition to Hodeida, they now are training Yemeni pilots on Sukhhoi fighter-bombers in Sanaa and on Mig17s in the southern city of Taiz.

In addition, Moscow is flooding the country with offers of scholarships and training courses in the Soviet Union. Reportedly about 1,500 Yemenis, most of them officers and military personnel, are now participating in various programs in the Soviet Union now, one-third more than three years ago.

By contrast, only about 60 Yemeni officers have been trained to date in the United States, while scholarships available to Yemenis number about 25 a year, according to U.S. Embassy sources.

Most important to the survival of the Saleh government as the guerrilla war escalates, the Soviets also have provided most of the weapons to the North Yemeni armed forces. They include 650 tanks, 450 armored personnel carriers, three squadrons of Migs and one of Sukhois, most of Yemen's helicopters and a wide variety of artillery.

The biggest Soviet arms deal to date came shortly after the United States poured in nearly $400 million of Saudi-purchased arms--a squadron of F5Es, 64 M60 tanks and 100 armored personnel carriers--in March 1979.

The United States presumed that President Saleh was planning to cut his ties to Moscow and replace Soviet arms and advisers with American ones. This at least was what William R. Crawford, then deputy assistant secretary of state for Near Eastern affairs, assured a skeptical Congress in March 1979 when the Carter administration asked for a national security waiver to send arms to North Yemen.

But later the same year, Saleh, disappointed over Saudi delays in delivering the U.S. arms, turned to the Soviets and concluded a deal that congressional sources placed at "well in excess of $1 billion."

The shock to Washington of this Yemeni-Soviet accord apparently cooled the ardor of the Carter administration for sending additional arms to Sanaa and also may have torpedoed the development of a major economic or military assistance program here.

Thus three years after proclaiming to Congress the vital importance of North Yemen to Saudi and American interests, the United States is providing this country with only $25 million to $30 million annually in economic assistance and has just started a military sales program worth $11 million.

"In private we call it peanuts," said North Yemen's Yale-educated Prime Minister Abdel Karim Iriani in a recent interview.

Iriani said the United States and Saudi Arabia had miscalculated in 1979 in assuming that North Yemen was ready to cut all ties to Moscow. "There was no intention of breaking with it," he said.

North Yemen's purpose in obtaining American arms, Iriani added, was to send "a signal to everybody that Yemen intended to have a diversified arms policy . . . in order not to create a dependency on one country."

Iriani described relations between North Yemen and the United States as "very good," but added that it appeared to the Yemenis that U.S. interest in the area fluctuate with the state of East-West tensions.

"When they the Americans perceive a polarization is taking place in the region, they become interested," said Iriani. "When that expectation becomes less significant, it seems they become less interested."

Since President Reagan came to office 14 months ago, no high-ranking State Department or administration official has visited the country for talks.

The attitude of the Saleh government toward the guerrilla war has been to say as little as possible. In fact, authorities here act as if the fighting does not exist.

This posture was brought home when a Ministry of Information official insisted that all questions regarding the front and its activities submitted to government officials be dropped as a possible subject of inquiry.

"There is no such thing as a front here," remarked the official with great politeness and charm.

In fact, there is very little overt evidence of the front's existence along the main road from the capital to Taiz in the south. The only hint of the fighting was in hospitals, which these days are full of victims of land mines laid by the guerrillas along mountain trails near Ibb.

That all may not be as well as the government insists became clear when a request to visit a village on Baadan Mountain east of Ibb was rejected by authorities who cited "heavy rains." Other sources said the area was off limits to foreigners primarily because of the danger of land mines.

In the view of most outside analysts here, it is unclear where the fighting is leading to or how Saleh is going to deal with the increasing political and military pressure on his government.

"The pattern is so erratic and the results so nebulous, it doesn't make sense," remarked one frustrated Western diplomat.

Saleh, who came to power four years ago this July after the slaying of his predecessor, has proven himself far more agile in maneuvering among the shoals of Arab and superpower politics than most outsiders had anticipated.

But there is a feeling among some of Saleh's Yemeni supporters that he is coming perilously close to falling off the tightrope of nonalignment.

"The time of choice has come for North Yemen," said one Yemeni analyst. "I do not think it can afford the luxury of remaining neutral any longer. Either it has to go with the East or the West."