The management of Pan American World Airways is threatening to sell off part or all of its fleet, routes and facilities if unionized workers do not agree to immediate pay cuts.

In a letter to employees from Pan Am Chairman Thomas Plaskett, the airline said its board authorized management to sell "any up to all" assets, a spokesman said.Assets to put up for sale may include "route segments and divisions, maintenance and terminal facilities, aircraft, engines and other equipment," the letter said.

"Such actions will be taken without regard to remaining fleet size, number or geographical coverage of markets, or number of employees," the letter said. "This may be shocking or frightening, but frankly, it should not be surprising."

The Pan Am spokesman, Jeff Kriendler, said the airline's board took the action because "labor has not delivered" on its part of agreements reached last winter on wage and work rule concessions that would have provided the financially troubled carrier with $180 million in annual savings.

But Kriendler dismissed as "absolutely not true" rumors Pan Am was considering filing for bankruptcy.

Pan Am has been negotiating with five labor unions representing its employees since their contracts expired Dec. 31, 1984. The airline said it has reached final agreements with just two of the unions, the Air Line Pilots Association and the Flight Engineers International Association, that it said will save it $75 million a year.

The airline unilaterally imposed pay cuts and work-rule concessions worth another $35 million a year on 4,100 employees who are members of the Teamsters union.

Neither 4,000 flight attendants nor 5,500 Transport Workers members have signed concessionary contracts initially approved in January, however. Those agreements would have provided another $58 million in annual savings for Pan Am.

Margaret Brennan, president of the International Union of Flight Attendants, said her members "overwhelmingly" rejected the proposed contract.

Brennan, who said she is "taking the letter at face value," said it must be regarded as "a real plan to liquidate" the airline.

"In this industry, you really hurt your business by coming out with something like this - and they've been very public about it," Brennan said. "I assume from this they've opened up the auction."

Spokesmen for the other four unions were unavailable.

In the letter to employees, Plaskett said Pan Am already plans to end its leases on at least three Boeing 747 jets, close its San Francisco flight service base as of Oct. 1, and cut flight operations in the last three months of the year "far beyond the normal seasonal reduction following the summer peak."

Plaskett said the airline has "much potential" and is "building momentum which could result in a turnaround at Pan Am," but still has to cut costs further.

"While the summer of 1988 will most likely be satisfactory, our planning must be focused on producing much better results in the winter than we ever have before," the letter said.