Federal mediators continued today to seek a solution to the five-day-old United Airlines' pilots strike while other airlines reported sharp increases in business.

Negotiators for the Air Line Pilots Association and United management have been unable to reach agreement in two days of talks at a suburban Chicago hotel, but will continue Wednesday, National Mediation Board Chairman Helen M. Witt said tonight.

What has happened so far, she said, "is very hard to characterize" as progress, but "new ideas keep popping up" and "both sides are reaching out" on the issues separating them.

That was a more optimistic appraisal than the one from Capt. Roger Hall, chairman of the United pilots group, who said in an interview that several proposals from the pilots have not produced "a lot of progress" and that "there has been absolutely nothing from the company" in the way of a modified offer.

The major issue is United's proposal to pay lower wages to newly hired pilots than to current employes. The ALPA is willing to accept a limited "two-tier" pay scale, but wants the new and the old tiers to merge in relatively few years; United wants the two to remain separate for many years.

United continued to operate about 15 percent of its daily flight schedule using management pilots and a few picket-line crossers. The ALPA said about 4 percent of its 4,900 striking pilots -- 200 pilots -- have crossed the picket lines.

The Federal Aviation Administration, meanwhile, was investigating at least 12 incidents in which air traffic control radio messages to or from United flights were blocked, apparently by other pilots "keying" their microphones to generate static or muttering on-the-air obscenities.

The incidents -- reminiscent of the early weeks of the Continental Airlines strike in 1983 -- caused FAA Chief Donald D. Engen to send a telegram to ALPA President Henry Duffy and United Chairman Richard J. Ferris warning that the FAA would pursue criminal sanctions in such cases and stating that the blockings "appear to come from pilot crews." Engen cited the potential safety hazard of a blocked air traffic message.

Meanwhile, such competitors as American, Western and Continental reported sharp traffic increases attributable to the United strike.

Linda Dozier of Western said "our Sunday business was up 30 percent" and more than 80 percent of the seats on Western's flights were full. Western added flights between the West Coast and Hawaii to help pick up the United slack. Pan American World Airways also added a flight between the mainland and Hawaii.

American, United's biggest competitor, reported that its traffic on Monday was 23 percent better than projected. Referring to United's chairman, an American Airlines executive joked, "We're thinking of sending a thank-you card to Dick Ferris."