Saudi Arabia is refusing to renew contracts for deliveries of U.S. manufactured arms to North Yemen and is threatening to cut off or reduce other vital aid altogether in an effort to persuade its Arabian Peninsula neighbor to distance itself from the Soviet Union, diplomatic sources here said today.

The action of the Saudis contrasts with what diplomats and Middle East analysts had been as relative Saudi neglect of North Yemen in the past year while the Sanaa regime drew closer to its own leftist opposition and the Marxist government in South Yemen.

At least 70 American military trainers were reported to have been dispatched to North Yemen in a $390 million emergency U.S. military aid package last year, but U.S. military sources say there are only two Air Force officers left in the F5E jet fighter training program.

The prospect of the permanent departure of the American advisers and the apparent willingness of North Yemen's military leadership to change course have now raised the possibility that the U.S. weaponry could end up pointed at the very Saudis who pressed Washington to rush it in during fighting last February along the border between the two Yemens.

Instead of responding to Saudi pressure by cooling relations with South Yemen and the Soviet Union, the North Yemenis appear to be edging closer than ever toward reconciliation with the Marist government to the south.

One European diplomat who follows Yemeni affairs closely commented on the Saudi aid ultimatums by observing, "The Saudis came down too hard."

If North Yemen is weaned away from the Saudis and the West, it would represent a major setback for American efforts to contain Soviet influence in the Middle East, right in the backyard of the world's leading oil exporter.

In balmier times, the loyalties of North Yemeni President Ali Abdullah Saleh and his relations with the doctrinaire Marxist leader of South Yemen, Abdul Fattah Ismail, would concern chiefly Arabs, area specialists or military strategists eyeing the Bab al Mandab Strait linking the Red Sea and Indian Ocean.

Since the Iranian revolution on the other side of the Arabian Peninsula and the confrontation with Moscow over Soviet intervention in Afghanistan, however, the direction of North Yemen's political leanings has taken on broader significance.

Knowledgeable diplomats here say the Saudi royal family -- Washington's main Persian Gulf ally -- is deeply concerned at the possibility of extended Soviet influence along its southern border.

An outright alignment with the Soviet Union by North Yemen also would be seen elsewhere in the Arab world as another loss of face and influence by the United States on top of Iran and Afghanistan.

Only a year ago, President Carter used emergency powers to speed U.S. jets and tanks to North Yemen in what was depicted then as a show of resolve against the Soviet Union in response to Saudi urging and a chance to gain North Yemen's loyalty at the strategic southern tip of the Arabian Peninsula.

Now, however, North Yemen is not only receiving Soviet Mig21 fighters and T55 tanks, but also showing increasing signs of reconciliation with Moscow's faithful allies in Aden, site of the region's main Soviet military base.

The American weapons -- 12 F5Es, about 90 M60 tanks and a similar number of M113 armored personnel carriers -- arrived mostly on schedule but far too late to help in the three-week February border war with South Yemen that was the occasion of their dispatch, diplomats said.

The threat of change in Sanaa was real enough to prompt a secret visit to the North Yemen capital last month by a senior member of the Saudi royal family. He reportedly laid down the ultimatum of reduced or cut off aid -- half the budget -- unless the warming up to Moscow is chilled again.

Although not involved in the threatened aid cutoff, another key factor in Yemeni-Saudi relations is that approximately 1 million of the 7 million, North Yeminis work abroad, most of them in Saudi Arabia. They send home $1.5 billion a year, a major share of Sanaa's foreign exchange earnings.

To add weight to their warnings, the Saudis already have held back on several aid payments, resulting in the late payment of December and January salaries for North Yemeni government employes, the diplomatic sources said. In response, Arab sources reported, Saleh has called back North Yemeni students in Saudi universities and resolved to seriously consider the attempts at reconciliation with the Marxist south that were part of an Arab League-sponsored truce last March.

Apparently relfecting Saleh's new willingness to deal, the head of the National Democratic Front of North Yemen, Sultan Ahmed Omar, announced Sunday in the South Yemeni capital of Aden that he and Saleh have agreed to form a new coalition government in Sanaa.

Omar, who is allied with Ismail's Marxist government in Aden, called the accord "a major turning point" in relations between the two Yemens. He said it coincided with agreement to hold elections, draw up a new nonaligned constitution and renew discussions on unification of the armed forces in north and south.

Omar and his chief deputy, Yahya Shami, spent 10 days in Sanaa last month in unannounced contacts with North Yemeni officials, apparently working out the agreement announced Sunday, diplomats said.

Analysts pointed out, however, that he announced it from the South Yemeni capital of Aden without specifying when it will be carried out, and that Saleh made no similar announcement from his own capital in the north. In addition, they recalled, the history of relations between the two antagonistic regimes is full of unity and reconciliation declarations followed only by continued hostility, border fighting and assassinations.

In the meantime, officers in the largely Soviet-trained North Yemeni Army have become increasingly resentful of what they regard as Saudi tutelage. Particularly irritating, the diplomats reported, was the necessity to deal through the Saudis on the American arms.

The friction fit into a historical pattern of rivalry between the Saudis, who consider themselves the source of Islamic and Arab culture, and Yemenis, who have pretentions along the same lines.

"The Yemenis were just waiting for a chance to tip the balance the other way toward the Soviets," a diplomat with experience in Sanaa said.