Two of America’s biggest airlines said Monday that passenger traffic increased in May and a key revenue figure came in higher than a year ago.

The results from American and Southwest indicated that U.S. airlines are continuing to prosper from solid travel demand and stable although elevated fuel prices. And that is boosting the stock of both companies.

American Airlines Group Inc., which owns American and US Airways, said that traffic rose 2.1 percent in May as passengers flew 19.2 billion miles. Domestic travel gained 1.7 percent and Latin American travel jumped 6 percent from May 2013, helping offset weakness in American’s relatively small business of flying to Asia.

The company said that based on April and May results and a forecast for June, it expects that revenue for every seat flown one mile will rise by between 5 percent and 7 percent in the second quarter, compared with the same quarter last year. That is a closely watched statistic in the airline business, and it can rise when airlines sell seats at higher average prices.

Southwest Airlines Co. said that the same revenue figure rose 8 percent to 9 percent in May, compared with May 2013. Last week, Delta Air Lines Inc. reported a 7 percent gain for May.

Traffic on Southwest inched higher by 0.6 percent from a year ago, as passengers flew 9.4 billion miles last month.

Average fares have been rising in recent years as mergers have left fewer major competitors. The remaining airlines have limited the supply of seats, helping to keep fares higher. In May, American and US Airways increased passenger-carrying capacity by a modest 2.4 percent while Southwest and its AirTran subsidiary cut capacity by 1.5 percent.

The average flight on American and US Airways was 84 percent full, down from 84.3 percent a year earlier. The average flight on Southwest and AirTran was 83.7 percent full, up from 81.9 percent a year earlier.

In late-morning trading, shares of American Airlines were up 18 cents to $44.06 after hitting a record $44.43 earlier in the session. Southwest shares were down 4 cents to $27.55 but reached a 52-week high of $27.70 earlier in the day.

Photo Credit: A Southwest Airlines jet waits on the tarmac at Denver International Airport in Denver January 22, 2014. Rick Wilking / Reuters