United Airlines and the Air Line Pilots Association agreed last night to resume contract negotiations Monday in Chicago as the pilots continued their strike against the nation's largest carrier.

The strike began when negotiations collapsed in Boston early Friday, but the two sides agreed to try again at the request of Helen M. Witt, chairman of the National Mediation Board.

It is unclear whether the early resumption of talks means the strike will be short-lived, but Witt is known to think that if the one remaining issue cannot be settled in the next few days, there is a distinct possibility of a long strike.

While Witt worked at getting the talks going again, most would-be United passengers were able to find alternatives, according to officials at several key airports. United was able to launch 220 flights, about 14 percent of its schedule, up from 11 percent Friday.

Other carriers were quick to fill the void left by the airline, which normally carries 15 percent of U.S. air travelers. Pan American World Airways added a flight from San Francisco to Honolulu; World Airways offered an extra Los Angeles-Honolulu trip, and PSA and AirCal announced extra flights between San Francisco and Los Angeles.

Amtrak added cars to a train from Los Angeles to Portland, Ore., and Greyhound Bus Lines said it would accept United tickets.

The only issue is United's desire to lower the starting wage for pilots hired in the future, then keep them on a lower pay schedule for many years. ALPA estimates that, under United's last offer before negotiations broke down, the average new United pilot would draw $500,000 less over a 30-year career than a current pilot. The result, ALPA says, would be a lower-quality pilot and a divided union.

United says it needs to reduce pilot costs to remain competitive.

As the strike continued, both sides engaged in bits of gamesmanship. United spokesman Joe Hopkins said that the airline has "several hundred pilots available," more unionized flight attendants available than needed and that the unionized mechanics were working.

The pilots, meanwhile, capitalized on a situation in which 90 Chicago-area senior citizens were temporarily delayed in Reno because of the strike. When 70 of them landed in San Francisco to wait for one of United's few flights to Chicago, they were greeted by a committee of uniformed pilots and their wives, who offered them a spaghetti dinner in a union hall.

According to pilots' spokesman David A. Jewell, United officials also offered them dinner in a restaurant, but the senior citizens voted and went with the pilots.

A United spokesman said that, "All I know is that they kissed our Reno station manager good-bye" after he had arranged for their flights home.

Henry A. Duffy, ALPA's chairman, estimated that the strike is costing United $5 million to $10 million daily.

United's airline revenues were about $6.2 billion last year, or $17 million a day.

Duffy joined the pilots' picket line at Washington National Airport yesterday afternoon along with pilots from several other airlines.

Earlier, Duffy said in an interview that the solidarity of the striking United pilots "probably has exceeded our expectations." He said, "The leakage across the picket line has probably been minimal. Only 56 went across." ALPA represents about 4,900 active pilots at United.

Duffy said "the most pleasant surprise" is that only six of 500 new pilots United had trained crossed the picket line. Hopkins said United has offered jobs to the 500 and that, "Some have come to work for us, but we have not quantified that."

United Chairman Richard J. Ferris has said that, if necessary, he will hire replacement pilots and slowly rebuild the airline.

"I think he's got a fallback plan to do that," Duffy said, "so we have to take that seriously. But that's so painful, I've got to think there are other solutions. This continues to be the strike that nobody wants. The other side doesn't seem like they want it, and I guarantee we didn't want it."

The hiring of permanent replacement pilots, Duffy said, would muddy the water considerably. "We've got one issue right now, the pay scale. If they introduce permanent replacements, we've got two issues . . . . If there is a way to get this settlement, we need to get it done over the next two to three days."

Duffy said the airline would have difficulty "replacing 5,000 pilots. I acknowledge that if they're willing to take the pain for a year, they could do it, but the devastation would be heavy."