Defense Secretary Caspar W. Weinberger ordered sweeping changes to protect the nation's military secrets yesterday, including procedures restricting security clearances to U.S. citizens and tightening eligibility requirements for Pentagon and defense contractor officials with access to sensitive materials.

These officials are now required to report all contact with foreigners from adversary nations as well as personal travel overseas, and to submit to credit and past-employment investigations.

The changes also call for a "reliability program" to be established in which Defense Department and industry supervisors are responsible for evaluating each employe with access to secret information and for certifying his fitness for a security clearance, with periodic reevaluations.

The plan mandates safer methods of communicating classified information, tighter control of reproduction facilities used for sensitive data and "cleanout" days to destroy unneeded secrets.

The changes would cover an estimated 4 million Pentagon and defense contractor employes. They are among 48 specific revisions of military security procedures recommended in November by a special Pentagon panel formed in the wake of the Walker spy scandal. Weinberger put them into effect yesterday, augmenting three steps he approved earlier. Twelve other recommendations are still under review by the secretary.

The three earlier reforms expanded the Pentagon's lie-detector program, strenghtened controls over "crypto-access" communications materials and required thorough inspections of all units handling sensitive documents.

Weinberger's latest move extends beyond the Pentagon. The plan calls on the Justice Department to report criminal backgrounds of defense or contractor officials with security clearances. It also urges the Senate president and House speaker to provide "minimum uniform control" over classified materials supplied by the executive branch.

In ruling out Pentagon security clearances for immigrant aliens -- foreigners admitted to the United States for permanent residency but who are not naturalized citizens -- the plan notes that few agents have penetrated the Pentagon.

But more liberal policies that had permitted immigrant agents to receive secret clearances "undoubtedly increase the risk of that," the plan said.

New regulations aimed at defense contractors mandate reinvestigations every five years of employes with top-secret clearances, require detailed justifications for clearances and corporate scrutiny of the background of applicants for clearances.