United Airlines, facing losses from its strike by 5,000 pilots, yesterday announced it will terminate its pension plans to recapture nearly $1 billion in excess pension fund assets.

United officials described the action as unrelated to the 25-day-old strike, and said they would immediately create a new pension plan, at current benefit levels, for employes but not retirees.

The airline will also buy annuity policies for 59,600 employes and retirees to cover the benefits accrued under the plans being terminated. No date for the termination was announced.

After meeting its legal obligations by providing the annuities, United would still net $962 million that would be placed in a trust for corporate expansion, said John L. Cowan, United's executive vice president.

He said the move also was calculated to prevent a hostile takeover of United by "outsiders who might be tempted" by the pension funds.

The announcement was immediately criticized by the AFL-CIO and the striking Air Line Pilots Association.

Union officials said that United had used questionable accounting methods to build up the surplus, and that the pension termination, which federal officials said would be the largest in recent history, would deprive retirees of future pension increases.

United declared a $3.2 million loss in the first quarter of 1985, "but they were funneling vast amounts of money into their pension funds," said ALPA spokesman David Jewell. Jewell said union officials believe that the airline was using the tactic to understate profits for bargaining leverage.

"Right now they've got a billion bucks more than they need, but what happens when they are a billion bucks short [in their new pension plan]?" asked ALPA spokesman James Damron. "We have our attorneys looking at it."

"United wants a lot of money to break this strike, hire scabs to replace pilots and withstand a long period of low revenue . . . . This is a legal but highly unethical method," said Bert Seidman, the AFL-CIO director of occupational safety, health and social security.

United has not been increasing pension contributions, as ALPA alleged, but rather has been reducing them because it knew they were excessive, said spokesman Rob Doughty.

"Both our employes and our shareholders will benefit from the financial stability this action will provide us," Cowan said.

"We will use these funds to build this company" by buying new Boeing aircraft and buying Pacific routes from Pan American World Airways, he added.

United's pension plan would be the largest of 705 major corporate plans terminated since 1980. The terminations have yielded a total of more than $6.5 billion, according to David M. Walker, acting executive director of the Pension Benefit Guaranty Corp., the quasi-governmental corporation that administers pension termination insurance. The largest previous termination netted $400 million for Occidental Petroleum in 1983, according to the corporation.

Pension plan "reversions," which are regulated by the Employe Retirement Income Security Act of 1974, (ERISA) have recently become controversial as rising interest rates fattened pension funds and made them an attractive target of employers, according to congressional critics of the practice.

Sen. Howard M. Metzenbaum (D-Ohio), who has proposed a moratorium on terminations, called United's action an "outrageous ripoff" because retirees will remain at a fixed benefit under their annuities and will not benefit from future pension improvements.