SEATTLE (AP) _ Vietnamese soldiers helped load 72 tons of marijuana onto a ship off Danang, Vietnam, a Coast Guard officer said in a hearing for 18 men arrested in what officials call the largest West Coast maritime drug bust.

The Coast Guard cutter Boutwell seized the Panamanian-registered Encounter Bay on June 30 when it was 500 miles off the Washington coast. Coast Guard officials last week said the vessel was boarded because it looked suspicious.

The detention hearing, which continues today, was to determine whether there was probable cause to seize the ship and arrest the 18 crew members, who could face potential prison terms of several years to life.

Lt. Thomas Rogers testified Tuesday the Coast Guard had prior information that about 72 tons of high-grade Thai marijuana was loaded off Danang.

Rogers, the law enforcement officer aboard the Boutwell, said he didn't know whether the cargo was already aboard when the crew got on the ship.

Asked by defense lawyer Peter Mair what he knew about who loaded the ship, Rogers said, ''The only additional information I can recall is that the Vietnamese army was involved.'' He did not elaborate.

''I am shocked'' at the revelation, Mair said.

Assistant U.S. Attorney Jerry Diskin refused to comment after the hearing.

U.S. Drug Enforcement Administration officials in Bangkok, Thailand, say there have been reports of merchant vessels taking on drugs in Danang for shipment to international markets. In addition, there have been unconfirmed reports that impoverished Vietnam has been growing opium to pay foreign debts.

In a telephone interview Tuesday with the Seattle Post-Intelligencer, Be An, second secretary to the Vietnamese consul general in Mexico City, denied the Coast Guard account.

''The problem is this: Some people don't tell the truth,'' he said. ''They've been caught. They could say any old thing. Our government doesn't permit drug trafficking. We've never had an interest dealing with these drug (operations).''

Defense lawyers for the crewmen arrested on felony charges of intent to distribute marijuana have tried to prove the seizure, which the Coast Guard has called the largest maritime drug bust off the West Coast, was illegal and the crew's constitutional rights were violated.

Rogers testified he didn't know whether documentation was found on board that would clarify when and where various crew members boarded.

Defense lawyers argued that the U.S. government can't prove that the crew knew what was aboard the Encounter Bay and that there wasn't probable cause to stop the ship.

Rogers testified he believed the crew members were told their rights but didn't know whether the rights, probably read in English, were understood by the crewmen from Southeast Asia.

The Encounter Bay crew members are three Americans, one from England, one from New Zealand and 13 from Indonesia, Thailand and Singapore.

Howard Weitzman of Los Angeles, representing crew member Samuel J. Colflesh, 31, of Philadelphia, and other defense lawyers have questioned whether the Coast Guard had the authority to fire upon, board, search and seize the 180-foot converted oil rig supply ship in international waters.

U.S. Magistrate John Weinberg would not allow questions about the Encounter Bay's destination, saying it was irrelevant to the proceeding.

Attorney John Muenster, representing Terrance Albert Nolan, 36, of the United States, said the ship's destination was relevant, since part of the charges against the crew require that the government prove the ship had either left or was approaching a U.S. port, or was in ''U.S. Customs territory,'' a term that neither he nor Rogers could define.

Weinberg denied a request by defense lawyers for copies of all Coast Guard communications leading to the Encounter Bay's seizure, saying the question of jurisdiction on the high seas should be addressed at later hearings.