Federal mediators reported progress in coal contract negotiations yesterday, but a union spokesman said differences are still too great "even to think about a contract extension" to avoid a nationwide coal strike next Tuesday.

"They're beginning to talk about the troublesome issues . . . substance instead of form," said Wayne L. Horvitz, chief of the Federal Mediation and Conciliation Service, after a brief bargaining session between the United Mine Workers and the Bituminuous Coal Operators Association during the morning. He said the talks were "very constructive."

The bargainers spent the rest of the day in separate strategy sessions and planned to return to the negotiating table today. UMW President Arnold Miller, who has been absent since the stalled talks resumed Friday, plans to be at the sessions today, union sources said.

Many miners were expected to halt work after the last Saturday shift, although the present contract does not run out until 12:01 a.m. Tuesday. Horvitz said a contract extension has not been discussed but did not rule it out. "I'm working day to day, I take it one day at a time," he said after yesterday's talks.

Miller said earlier he would not consider an extension unless the talks were making "substantial progress." A union spokesman quoted Harry Huge, the union's chief negotiator in Miller's absence, as saying there was "movement" but added: "They [the two sides] are obviously too far apart even to think about a contract extension."

The coal talks have been so snarled that even discussion of major substantive issues, including the miners' near-bankrupt health and pension funds and the wildcat strike problem, amounts to a breakthrough of sorts. It wasn't until the mediation service stepped in last weekend that this began to happen and even then it took several days to resolve procedural and personality problems.

The UMW-BCOA agreement covers 130,000 miners who produce about half the nation's coal. Another 30,000 miners are covered by related contracts, meaning a strike would idle about 160,000 miners, mainly in Appalachia.