United Air Lines yesterday dramatically cut the advanced-purchase requirement on inexpensive supersaver fares in an effort to win back passengers lost because of the 29-day pilots' strike.

The measure was one of several announced by the carrier, the nation's largest, in a promotion drive industry analysts termed as restrained and unsurprising given the loss of traffic during the strike.

In an effort to capture the peak-season vacation market, United said passengers will be able to buy ultrasaver tickets -- which can mean up to 70 percent off normal fares -- only seven days before their flights. The offer is good through the end of June. The normal period is 30 days.

Other measures announced by United include:

A July 4th special that allows ticket buyers to get a 50 percent cash rebate if they fly during the first week of July, even if they bought tickets before the strike, which ended Friday.

Allowing members of Mileage Plus, the carrier's frequent-flier program, to receive free upgrades to first-class, on a space-available basis, until Aug. 15.

Awarding Mileage Plus passengers who pay full fare double credit for their flights.

American Airlines, United's chief domestic competitor, immediately announced that it will match all these measures, except for the cash rebate. Lowell Duncan, the company's vice president for corporate communications, said that American will offer all passengers who participate in American's frequent-flier program double credit, not just those who pay full fare, for miles flown before Aug. 15.

Spokesmen for Eastern and Delta airlines said their companies are studying the United measures and probably will announce a response today.

Industry analysts, however, said the United move is unlikely to provoke a fare war, as happened in 1979. That was the last time the carrier attempted to rebuild traffic after a major strike. At that time, United offered half-fare coupons to all fliers.

United normally has 1,550 flights each day, carrying 120,000 passengers. During the strike, the airline has been operating 223 flights daily.