Nine months ago, President Carter resorted to emergency powers to rush $383 million in American arms to North Yemen, a country he hoped would become a staunch friend of the United States in the volatile Middle East.

Today, there are increasing signs that the gamble is in jeopardy. Crates of Soviet Mig21 fighter planes are piling up in the North Yemeni port of Hodeida, indicating that the Soviet arms spotted coming into the country this summer marked just the beginning of a Russian comeback.

North Yemen, though barren of oil, is strategically important to oil-rich Saudi Arabia on its northern border and to the United States. This makes the increasing flow of Soviet arms into North Yemen worrisome to Riyadh and Washington.

Saudi Arabia, concerned by the large communist presence in South Yemen, has been willing until now to buy American arms for North Yemen.

"But the Saudis won't keep buying our arms if North Yemen accepts help from the Russians," an administration official predicted last night.

This, in turn, would mean that Washington would lose the leverage it gained in North Yemen, at Moscow's expense, by becoming that country's main source of modern arms and military training.

"Disturbing and regrettable," the administration official said in characterizing the new turn of events in North Yemen, where the Carter administration had placed such high hopes. But he said Carter's bet on the country is only in jeopardy, not lost.

U.S. Mideast specialists said the North Yemeni government is trying to demonstrate its independence from Saudi Arabia by getting some of its weaponry from the Soviet Union.

It is too early to know, they said, whether the Soviet arms going into North Yemen represent a temporary turn toward Moscow or the full-scale resumption of a relationship the Carter administration had hoped to break.

The administration's contention for the last few years has been that North Yemen was turning away from the Soviet Union, presenting a golden opportunity for the United States.

The State Department, in justifying to Congress its fiscal 1980 military aid package to North Yemen, said:

North Yemen "in the last few years has begun a process of reorientation away from the Soviets and toward the West."

This spring, South Yemen attacked North Yemen. About 1,000 Soviet military advisers and hundreds of Cubans and East Germans had trained the South Yemeni forces.

It was at this juncture that Carter decided to place a lot of chips on North Yemen in hopes of gaining a new friend in the Moslem world.

He signed an emergency "determination" March 7 that enabled the United States to rush planes, tanks and armored cars to North Yemen without waiting the customary 30 dyas for Congress to review the deal.

Twelve F5E fighter planes, 64 M60 tanks and 50 M113 armored personnel carriers went to North Yemen, at a cost of about $383 million.

This was the biggest part of $540 million in weapons and training the United States has sold to North Yemen since 1976. Saudi Arabia paid most of that bill.

For the reasons no one pretends to understand fully, South Yemen called off its spring war against North Yemen after some intense fighting. The United States proceeded to teach North Yemen to use its new American weapons. Two instructor pilots, for example, taught the North Yemenis how to fly the F5Es. The Pentagon says there are still 25 Americans in North Yemen training its military.

The Soviet Union, besides having 1,000 advisers in South Yemen, has hedged its bet by keeping 100 military advisers in North Yemen. This means that both U.S. and Soviet military advisers are training North Yemeni forces.

The F5E jet fighters the North Yemeni pilots have learned to fly are prestige items for an underdeveloped country. So are the Mig21s. If the military presence of both the Soviet Union and the United States continues in North yemen, the day is not far off when F5Es and Mig21s will off the same airstrip in that country, possibly to take on the Mig21s the Soviets have sent to South Yemen.

"Yeah, it's unusual," conceded one Pentagon official involved in the arms sales program.