An Iranian national living in Gaithersburg is indicted on charges of conspiring to smuggle to Iran spare parts for aging U.S. tanks, Cobra attack helicopters and jet fighters.

A Prince George's County man is sentenced to five years in prison and fined $110,000 after pleading guilty to selling microchips used in cruise missiles and antisubmarine warfare equipment to an Austrian firm that resold the merchandise, valued at more than $6.5 million, to East Germany, Poland and the Soviet Union.

Four New York men are accused of illegally shipping $2 million in "night vision" devices to Argentina during the Falkland Islands war, $200,000 worth of weapons to Iraq and attempting to export another $1 million in weapons to Poland.

These cases are among a growing number involving illegal exports of arms, related equipment and high-technology devices suitable for military uses, to the Soviet Union, its allies, other hostile nations or terrorist groups.

"It's just like any other crime where there are high payoffs," a law enforcement official said of the murky netherworld of black-market arms merchants. "As in drugs, there are people willing to take the risks."

Iran has been among nations most active in seeking weapons. Embroiled in a war with neighboring Iraq for nearly five years, it has been barred here from buying new arms or spare parts for a weapons arsenal supplied by the United States during the rule of the shah. The restriction came shortly after the seizure of hostages at the U.S. Embassy in Tehran Nov. 4, 1979.

Last week, six persons were arrested and charged with conspiring to smuggle $9 million worth of sophisticated antitank missiles to Iran. They included an Army artillery expert, a purported Iranian government official and a California man who allegedly told an FBI undercover agent that he had made more than $6 million on such deals in the last two years. A seventh suspect is at large.

Among items on a multimillion-dollar shopping list of weapons that two of those arrested allegedly presented to an FBI undercover agent posing as an arms broker were French-made Exocet missiles, F4 fighter jet engines, a mobile hospital and Harpoon, Sidewinder, Sparrow and Phoenix missiles.

The case followed the arrests last month of seven people, including three Navy employes, as part of an alleged conspiracy to steal spare parts for F14 fighter planes for sale to Iran. The accused smugglers allegedly used two fictitious companies as fronts, shipping parts from California to New York to London to Iran in boxes labeled medical supplies or automobile parts.

"Ever since the shah's government fell in January 1979 . . . their war machine has been falling apart, in part because it is getting old and rusty, and in part because they've been using it up against the Iraqis," Customs Commissioner William von Raab said in an interview.

Iran is far from the only participant in the booming black-market in munitions and high-technology items. Other major players involved include Libya, Iraq and the Irish Republican Army, according to Customs officials.

Forty-four corporations and 207 individuals were charged in 89 cases from January 1981 to last June considered "significant" by the Justice Department's Internal Security Section. The cases range from smuggling weapons for the Provisional Irish Republican Army to alleged export to Israel of 800 krytrons, electronic microswitches that can trigger nuclear weapons.

The bulk of those were the fruit of the Customs Service project "Operation Exodus," aimed at stemming the flow of munitions and high-technology material to the Soviet Union and elsewhere. According to Customs statistics, 607 people and corporations were indicted, and 369 convicted, from October 1981 through last March, and Customs inspectors seized arms and other goods valued at $274 million.

According to a State Department official involved in issuing licenses for legal arms sales, the most recent arrests reflect a heightened effort by the Iranians during the last 18 months to buy arms here. Iraq's advanced Soviet MiG23 and French Mirage fighter planes give it air superiority over Iran.

In the six years since the arms embargo was imposed against the government of Ayatollah Ruhollah Khomeini, von Raab said, Iran's acquisition techniques have grown increasingly adept. "In the beginning, they kind of stumbled around trying to buy things," he said. "Now, they've gotten themselves introduced to the major arms traffickers."

Iranian officials speak bluntly about methods they say they are prepared to use in the arms quest. "Some of the arms they need, they buy on the black market," Saeed Imami, spokesman for the Iranian Mission to the United Nations, said of his government's war effort.

U.S. officials expressed concern about weapons sought in the case that resulted in last week's arrests because most were self-contained missiles rather than spare parts.

"This is the first major effort which has come to the public's attention to acquire these self-contained missile systems," von Raab said. The worry, he said, is that such missiles are not only useful in the war against Iraq but could be used by Iranian-backed terrorist groups. "They're the kinds of weapons that the Libyans have been trying to get for terrorist purposes."

Another source of concern in last week's and the one involving F14 parts is the fact that, for the first time, military employes have been accused of participating in alleged smuggling conspiracies.

In the F14 case, agents traced the parts to the aircraft carriers USS Kitty Hawk, Carl Vinson and Ranger. Lt. Col. Wayne G. Gillespie, arrested July 31 in Alexandria, is accused of serving as a "missile inspector" for those attempting to buy 1,140 antitank TOW missiles.

"For the first time, it does appear we do have people in the military involved," said Roger Urbanski, director of the Customs Department's Strategic Investigations Division. "It concerns us greatly because . . . in the cases where they are stolen from the U.S. military, there is no question that they are of considerable military value."

The illicit trade works two ways. When high-technology material such as computer microchips is sought, a person known as a "diverter" often obtains the required export license by representing that the goods are to be used by an overseas company in a country given access to the technology. Then, often in multilayered transactions, the material is diverted to the Soviet Union or a Soviet bloc country.

When the trade is in weapons, the arms broker generally acquires them from a thief.

"It's all the same game," von Raab said. "The arms transfers are of a more immediate danger because there you're talking about arming terrorist government with usable weapons. In technology transfers, what you're talking about is arming the Russians in the future with sophisticated systems.