The announcement came as American Airlines announced it would lay off more than 3,100 flight attendants and Air Canada said it would suspend service to New Orleans for the summer.
United said the bulk of the added flights will be domestic although a number will involve Europe, Australia and Tokyo.
A spokesman said though revenue passenger miles were down 13.4 percent in April systemwide, they actually were up in North America and Europe.
Network planning director Marty St. George said the flights were added based on increased bookings "that indicate a strong recovery in many of the markets we serve." The decision will bring the number of daily flights to 1,722, up from 1,560 a day this month.
United plans to use its automated schedule change notification call-out system to notify passengers about changes in flight numbers and times.
"United will restore all transatlantic service cutbacks made in May and some service in the Pacific," St. George said. "However, because we are still seeing weakness in bookings to some Pacific markets due to SARS (severe acute respiratory syndrome), we have extended the spring frequency reductions on several Pacific routes."
He said Pacific bookings will continue to be monitored on a daily basis and a decision on altering the Asian schedule will be made in the fall.
The Association of Flight Attendants said the additional flights will lead to the recall of 1,527 flight attendants but the Air Line Pilots Association said no pilots would be recalled and 74 scheduled furloughs would go ahead.
"We still have an excess of pilots (in the company's view)," said Dave Kelly, a spokesman for ALPA's United chapter. "As the company implements the new contract, (the company still believes) we will have more pilots than we need. The best we can hope for is this news will help (mitigate) some of the future furloughs."
United filed for bankruptcy Dec. 9 and lost $1.3 billion in the first quarter, blaming a shaky economy, tensions over Iraq and stiff competition from low-fare carriers.
United's unions have ratified new contracts designed to save the carrier $2.56 billion annually.