Whatever progress was being made in the current round of negotiations between the National Football League's owners and striking players apparently was undercut yesterday when the talks hit two major snags.

Any hope of ending the 18-day-old players strike this weekend was dashed when the owners, through chief negotiator Jack Donlan, shocked union chief Gene Upshaw by demanding a fundamental change in the players' pension system and insisting that the new collective bargaining agreement run for at least six years.

After a dinner break, the talks resumed shortly after 11 p.m. and were scheduled to continue into the night. It was not known if there would be negotiations Saturday.

Upshaw and union officials had been encouraged by the pace and tone of the bargaining on Thursday. But yesterday's events led Doug Allen, assistant executive director of the NFL Players Association, to accuse management of stalling in the negotiations so that another weekend of replacement games could be played, thereby further weakening the union's bargaining position.

After John Jones, spokesman for Donlan and the NFL Management Council, announced that replacement games will be played Sunday and Monday, Allen said it wouldn't surprise the union if Donlan broke off negotiations sometime Saturday until early next week.

"The insistence with which the owners are pushing this idea of a six-year agreement," said Allen, "and now seeking to make this radical change in the pension system, which is repugnant to the players, is obviously an excuse to push through this weekend with scab games."

Allen said that the owners declared last month that if the union took the issue of free agency off the bargaining table, it would take little time to reach a new collective bargaining agreement. But now, he said, after the union has relented on free agency, "It's obvious that free agency, as a stumbling block for settlement, was just a smokescreen."

Several owners have indicated they want to play at least one more replacement game, then wait to see how many players cross picket lines on Monday, Tuesday and Wednesday of next week. Already, 129 union members (of 1,600) have returned to work and will be eligible to play Sunday and Monday.

While eight minor issues have been initialled by Donlan and Upshaw, the major ones remain in contention. And this new dispute over the pension system could be an even bigger obstacle to reaching an agreement than free agency has been.

The owners are saying they will continue to guarantee the players' benefits, and even increase those benefits. But Donlan, during a fourth straight day of negotiations with Upshaw in Virginia, said the owners no longer want the amount they contribute to that pension fund to be specified in the collective bargaining agreement.

The amount of the contributions from the league's owners has been specified in every collective bargaining agreement since 1968. The union wants that to continue.

Allen was adamant about the pension system remaining the same. "This is the fundamental benefit provided through the union negotiations to the players," he said.

The owners feel that any surplus (interest that accrues as a result of high investment performance) belongs to them, not the players, who would have already received their benefits in full. The union contends that any surplus should go to the union, as it has since 1968.

The union and management already have filed suit against each other over $18 million the players claim the owners promised during the 1982 negotiations to place in the pension fund, but have not.

The owners promised to pay $12.5 million per year into the fund; but player benefits each year have only cost between $7 million and $8 million. The owners say the remainder (the $18 million, spread over the five years) was not tax deductible, and therefore they could not pay it into the fund.

There is currently a $34 million surplus in the pension fund, the union says.

"We're not asking for anything more than we've already had since 1968," Allen said. "That's a demand unheard of in labor negotiations. It's a tactic designed to derail the negotiation process and we're very disturbed by it. We're not about to give this up. There's no older tradition than the contribution to the pension plan -- as specified in the agreement -- from the owners to the players."

The union was nearly as upset about management's demand for a six-year agreement, instead of the three-year contract the two sides agreed on six weeks ago. The management proposal would allow the union to renegotiate economic issues at the end of three years; if the two sides disagreed, the matter would go to an arbitrator.

Allen said a six-year contract is "not a good labor practice, and not typical of other management-labor contracts. It's incredible, that the one thing of substance we agreed on would be subject to change at the whim of management."

But Jones countered that management had told the union it would agree to a three-year deal, "only if there is not a strike, and clearly there is" and that the desire for a six-year contract is the owners' effort to find a prolonged period of "labor peace."

The strike could now wind up lasting much longer than the union expected. And it is also becoming clearer that the owners, with their replacement games enabling many or all of the 28 franchises to turn a profit despite the strike, might seek to prolong that strike.

Some owners reportedly are hoping for an immediate end to the strike, while at least a few, according to sources, are intent on teaching the union a lesson.

The owners who want an end to the walkout are hoping any delay in negotiations doesn't backfire against the clubs.

"They are intent on continuing these scab games for one more embarrassing week," Allen said. "To continue this would do irreparable harm to the integrity of the league, and its ability to bargain with the television networks. What does {NFL Commissioner} Pete Rozelle say when he goes in there in three years to negotiate a new television contract? He'll be in a terrible bargaining position."

Donlan and the owners, through Jones, insisted that, "There is no dragging of the feet. I think it's clear we're here to get an agreement, if it's attainable," Jones said.

"We didn't say it would happen in the blink of an eye. We said {Thursday} when the union expressed encouragement that it was an overstatement. There's no desire or attempt {by management} to make this an abbreviated process."

But Jones added that Donlan sees no purpose in continuing the talks "if there is no substantive movement" on the major issues. "There is no timetable I can offer," he said.