Vice President Bush revealed yesterday that President Reagan signed a secret national security directive two months ago that defines drug trafficking as a national security threat and allows stepped-up use of military force to stop the narcotics flow.

Bush took the unusual step of disclosing the directive to make "every American understand a very real link between drugs and terrorism." He charged the Sandinista government in Nicaragua with using money from illegal drugs to finance international terror, and he blamed Cuban President Fidel Castro for harboring airplanes used in drug smuggling.

Bush also alleged a drug connection behind the 1985 assault by M19 guerrillas on Colombia's Palace of Justice, in which 100 people were killed, including 12 Supreme Court justices. He said Colombian authorities discovered after the siege that the rebels had destroyed all U.S. extradition requests for Colombia's major drug traffickers.

Congress last year directed the military to take an expanded role in the nation's war against drugs, but the new agency-wide directive "will officially authorize that use on a more formal basis and should increase it," according to Bush press secretary Marlin Fitzwater. "This National Security Decision Directive allows us to increase the use of military force."

Until now, the Coast Guard has had the primary military role in intercepting drug traffic, with the other services mostly providing communications and surveillance equipment for law enforcement officials, Fitzwater said.

But that limited role has led to some concerns that military involvement could be politicized, that there could be pressure for domestic surveillance of civilians and that military personnel data banks could be opened during court cases.

Bush said the administration is considering using military equipment such as aerostat balloons -- basically, blimps with radar -- to detect aircraft over the U.S.-Mexican border. The military would not be used to make arrests, he said, but the military presence would deter traffickers.

Bush noted that military officials have resisted using their forces to combat drugs. "You do encounter a justifiable concern on the part of the military in terms of their readiness," he said.

National Security Decision Directives are rare, top-secret and usually deal only with the most pressing national security concerns. The directive linking drugs and terrorism was signed April 8. Bush, who chairs an administration task force on drugs, released a declassified version at a news conference in Houston, where he was meeting with law enforcement officials.

The administration has announced various initiatives on drug trafficking in the past, and the charges of Nicaraguan and Cuban involvement in the drug trade -- which those governments have denied -- are not new.

But there has been division within the administration over how strongly to link Managua to narcotics trafficking.

While various administration officials have alleged instances of drug money being used to buy guns, or cited violence inspired by drug trafficking, the directive appears to establish the first official policy linking drugs and terrorism.

During a recent tour of Latin America, Bush showed government officials in Colombia, Honduras, Costa Rica and El Salvador a U.S. intelligence photograph that depicted drugs being unloaded from a plane in Nicaragua. In the photograph was a man Bush identified as a deputy to the interior minister of the Sandinista government.

Charges that Colombian guerrillas who stormed the Palace of Justice were after drug records surfaced Nov. 8, shortly after the 27-hour kidnaping and siege. Senior government officials there said then that the rebels' aim was to destroy records in U.S. extradition requests against about 80 drug traffickers who may have funded the guerrillas.

But that charge has been disputed in Colombia. After the siege, Supreme Court Judge Humberto Murcia said the rebels "never mentioned the narcotics records."