U.S. military personnel will be allowed to help U.S. agencies and foreign governments plan assaults on narcotics traffickers, equip police forces and transport them to attack sites under a new presidential directive that defines drug traffic as a threat to U.S. national security, administration officials said yesterday.

The secret National Security Decision Directive, revealed Saturday by Vice President Bush, will also permit the armed forces to dedicate personnel and equipment such as radar-equipped airplanes or satellites to fighting drug traffic, the officials said.

Military assistance against narcotics has been limited mostly to training U.S. and foreign personnel and to providing temporary loans of equipment on a space-available basis, in part because military leaders have been wary of allowing their forces to be turned into police officers. Defense Secretary Caspar W. Weinberger once called the idea "very dangerous and undesirable." Until 1981, even that level of aid was against the law.

Now the armed forces will be able to help in almost any area of drug law enforcement except arrests, seizure of materials and apprehension of suspects, as long as their primary defense mission is not jeopardized, the officials said.

A major effort last year to disrupt drug transport in the Caribbean, called Operation Hat Trick II, used military surveillance aircraft to spot smugglers and track them for foreign governments and the Coast Guard. The operation is likely to be expanded under the new directive. Other new activities aimed at breaking the reported link between narcotics traffic and international terrorism are still in the planning stages, the officials said.

President Reagan signed the directive April 8 after nearly a year of interagency studies and recommendations from the Joint Chiefs of Staff, among others, that the antidrug role of the U.S military could be expanded, according to the officials.

"For the first time, the U.S. government specifically states that the international drug trade is a national security concern because of its ability to destabilize democratic allies through the corruption of police and judicial institutions," Bush said in making public a declassified version of the directive. He also said "a very real link between drugs and terrorism" exists.

But the directive "stops short of making drug interdiction a mission of the Defense Department" and leaves civilian law enforcement agencies in the lead role at home and abroad, said Howard Gehring, director of the vice president's National Narcotics Border Interdiction System, an interagency drug control effort.

Morton H. Halperin, director of the Washington office of the American Civil Liberties Union, said he is concerned that involving the armed forces in drug control might lead to covert activity -- such as surveillance of U.S. citizens -- that would be illegal under ordinary law enforcement procedures. "The military should stay out of law enforcement. They're not trained in it, don't know about constitutional rights . . . . There are none of the normal checks," he said.

If Reagan wants additional capability to fight drugs, "he should ask Congress to fund it for the regular agencies. That way there would be a public debate over whether the changes are in the public interest," Halperin said.

Barry W. Lynn, an ACLU legislative counsel, said military involvement could lead to chain-of-command problems and to difficulties in presenting evidence that may have been obtained by covert means at drug trials. "You can't hide the role of military officials in gathering evidence once a case goes to court," he said.

The existing level of military involvement was permitted by a 1981 revision of the post-Civil War posse comitatus act that had barred the armed forces from all domestic law enforcement.

Lt. Col. Pete Wyro, speaking for the Defense Department, said the Pentagon is allowed to pass along to drug agencies information it obtains while conducting its normal business, such as telling them the path of a "low, slow plane in an area known for drug traffic." Other officials said spy satellite information is also provided routinely.

The 1981 act also allows the loan of equipment such as Army Blackhawk helicopters to the Drug Enforcement Agency, which provides its own pilots and maintenance, and the training of drug agency and foreign personnel in using borrowed radar, night vision devices, cryptographic equipment and helicopters, Wyro said.

U.S. Navy vessels carrying Coast Guard teams have intercepted suspect boats, held them while the Coast Guardsmen searched them and made arrests, and then towed the boats to shore, he said.

"It is fair to assume" that all these activities would increase under the new directive, he added.

Wyro said current law requires that all requests for assistance be weighed for their impact on the fundamental military mission. According to Gehring, this requirement stands under the new directive, but narcotics traffic is now "a higher priority for assessing defense needs."

The directive lists six areas of expanded action:

*Foreign assistance planning: "A new door is opened" for military aid for civil law enforcement agencies abroad, although "some conditions" will be attached to avoid abuses such as those that have occurred in the past when U.S. aid was involved in coups or human rights violations, Gehring said.

U.S military aircraft can be used to transport foreign police "only if U.S. pilots are on board . . . and for very limited uses," he said. U.S. personnel involvement in "life-threatening situations" will be kept at "an absolute minimum," he added.

*Supporting counternarcotics efforts: Regional or hemispheric antidrug operations bringing together U.S. and allied forces could be planned in the Pentagon. Communications equipment for detecting incoming traffic and security for the equipment, such as coding techniques, is "more likely" to be provided than armaments, Gehring said.

*International talks: "Drugs will now be taken into consideration in any bilateral or multilateral agreements," he said.

*Intelligence efforts: Drug-related work will become a priority instead of "being fitted in somewhere," Gehring said.

*Communications: Scrambling, jamming and codes will keep interagency communications secure.

*Promoting foreign control and education programs.