A federal mediator reported progress in contract negotiations between United Airlines and its pilots' union yesterday, but the primary issue in the dispute remained unresolved.

The pilots have said they will strike at 12:01 a.m. Friday EDT if there is no settlement.

United, the nation's largest airline, with more than 1,500 flights daily, says it will keep flying with a combination of management pilots, about 500 trained but not-yet-hired crew members and Air Line Pilot Association members who cross their own picket lines. United has about 4,900 working pilots.

However, it is clear that, if there is a strike, United will not be able to operate its full schedule and that many travelers will have to find other airlines.

Helen M. Witt, chairman of the National Mediation Board, said at a news conference in Boston yesterday afternoon, "We sense some progress at this time. We don't have an agreement as yet, but we continue to feel optimistic." Witt said that "We still have an issue of new entry rates."

United wants to pay newly hired pilots less than it pays its current flight crews and to continue the two pay schedules for many years. The ALPA says it is willing to permit two schedules, but wants them to merge in five years.

Both sides intensified public relations campaigns yesterday. The pilots have set up a series of satellite-connected television rallies for tonight to whip up enthusiasm among the troops if a strike occurs. "We're ready; we've been planning for 18 months," said Capt. Sam O'Daniel, one of the pilot spokesmen.

Capt. Roger Hall, leader of United's ALPA group, has charged that the so-called "two-tier" pay schedule would lower the overall quality of pilots at United.

Richard J. Ferris, United's chairman and president, said United's proposed new-hire salaries "are not demeaning to the pilot profession. These are honorable salaries that will enable us to attract qualified airmen now and in the future."

Ferris said that a beginning United pilot, hired as a second officer, could expect to earn an average $32,000 to $35,000 annually over the next 10 years under his proposed new pay schedule, and that a captain with 20 years' service or more would average $110,000 annually over the next 10 years.

Additionally, United has offered most of its existing flight crew members a 6 percent increase effective immediately and an additional 2 percent in 1986.

A two-tier schedule has been negotiated with a different pilots' union at American Airlines, United's major competitor. Financial analysts are of the opinion that United must succeed in its negotiations to remain competitive.

John Pincavage, an analyst with Paine, Webber, said, "I think they have to win it simply because American Airlines has gotten all those concessions. If you look at the difference between United and American, it's all labor costs."

The ALPA, which has pending negotiations with Delta Airlines and other major carriers, is concerned about establishing what, from its point of view, would be too generous a precedent.

United has been running advertisements in pilot-heavy cities offering to hire fully qualified captains for $75,000 and first officers for $50,000, which is more than is paid at such United competitors as Continental or People Express. United says that 5,500 pilots have applied and can be used to rebuild the airline's cockpit crews if there is a strike.

United also has been encouraging flight attendants and mechanics to keep working, and Ferris promised yesterday that "We will never negotiate away the rights of pilots or any other employes who help to keep this airline flying."