U.S. District Court Judge John H. Pratt issued a temporary order yesterday barring a nationwide postal strike that has been threatened for early this week.

Union leaders were noncommittal about whether they will obey the order, and knowledgeable sources said wildcat disruptions are possible even if an official strike is not called.

Within minutes after Pratt signed the six-day restraining order at the fovernment's request, chief federal mediator Wayne L. Horvitz was on the telephone in an attempt to resume talks between the Postal Service and the three postal unions whose members rejected a proposed new three-year contract last week.

The two largest unions, the American Postal Workers Union and the National Association of Letter Carriers, have mandated their officers to call a strike if the Postal Service has not reopened contract negotiations within five days from the time the contract was rejected. The strike deadline is midnight tomorrow for the NALC and Wednesday for the APWU.

Postal Service officials, relying on a federal law that prescribes arbitration when a bargaining impasse occurs, have refused to renegotiate the contract despite appeals from both horvitz and union officials for a negotiated settlement.

Talks aimed at finding a way out of the impasse broke off early yesterday after a four-hour session in Horvitz's office that was described by participants as "tense" and "grim." Postal Service officials were reportedly angry at the unions for leading Pratt on Friday to believe that the talks might avert a strike.

Pratt refused to grant the government's initial request for a restraining order Friday but said he would do so immediately if the talks broke down --from his home in Chevy Chase.

He said the unions argued that a restraining order was premature but "there was no question about the matter of a strike being contrary to law." Although federal law bans postal strikes, one occurred in 1970 when about 200,000 postal workers walked off their jobs for eight days, receiving both amnesty and substantial pay raises when they returned to work.

However, this year the Postal Service has taken a harder line, firing more than 100 workers who participated in wildcat walkouts after the tentative pact was negotiated last month and seeking an injunction that exposes both union officers and members to[WORD ILLEGIBLE] if they defy it.

The Postal Service has strikecontingency plans -- one of them called "Operation Graphic Hand" -- that include use of federal troops to handle the mails, curtailed service and use of private firms to assist in mail delivery.

The situation presents the APWU and NALC officers with a no-win predicament: defy the law and risk jail or defy official directives of their own unions and risk internal reprisals.

Asked whether he would obey the court order, APWU President Emmet Andrews said he couldn't say "because they'd close in on us if I did." He said Friday, before the government sought the injunction, that he had "every intention of fulfilling that (the union's) mandate."

An APWU spokesman said last night that Andrews' Friday statement still holds, meaning a strike remains possible unless bargaining is resumed.

NALC President J. Joseph Vacca had no comment as of late yesterday, but an aide said he was unlikely to commit himself before tomorrow.