CHICAGO — The most expensive seats on United Airlines' international flights will get an upgrade beginning next year, as the airline tries to lure more business and international travelers.

The airline will spend $165 million over three years on new first- and business-class seats, United announced. The Elk Grove Township, Ill.-based carrier would not reveal what improvements it planned but said it had hired B/E Aerospace Inc. to design and build the better seats.

"In the international markets ... the first- and business-class customers are about half the revenue on the airplane," said Dennis Cary, United's senior vice president of marketing. "These are customers who for a 12-hour journey are looking for a different experience."

The money for improved seating is separate from a $400 million capital spending plan the airline recently announced to improve cabin interiors, upgrade computer systems and purchase new ground equipment.

United's decision to spend money retrofitting the seats on its planes brings it in line with similar investments being made by foreign carriers, said Amin Khoury, chairman and chief executive of Wellington, Fla.-based B/E Aerospace.

"The program announced today is particularly gratifying, as it demonstrates a competitive response from a large North American carrier to invest significant resources to begin to upgrade its fleets," he said in a statement.

The first of the new seats will be installed in early 2007, United said. Three aircraft types — the 747-400, 777-200 and 767-300 — will get the new seats.

Better seating, airlines believe, is something customers are willing to pay for. Some United flights already feature first-class seats that lie flat for those who want to sleep or have room to function as a minioffice.

Airlines have found even those in coach are willing to pay more for a better seat. United sells upgrades to Economy Plus, a slightly roomier seat in the coach section.

This week, Northwest Airlines announced plans to charge $15 extra for seats on the aisle or in the exit row.

The decision to upgrade premium seats on international flights appeals to two customer bases that airlines are aggressively pursuing.

Flights across the ocean generally command higher ticket prices, prompting many large carriers to focus more resources on those routes.

They also are working to attract the loyalty of first- and business-class customers, groups of travelers who generally are not as fixated on getting the lowest possible price as those traveling in the economy section.

Which airline has the seat that is most comfortable to sit in and work at is something business travelers consider, said Kevin Mitchell, chairman of the Business Travel Coalition, a Pennsylvania-based organization that represents corporate travel buyers.

"Business travelers are the most important of the revenue-generating customers of the airlines," Mitchell said.

"There has been a trend to strip things away from the economy passengers and reward the highest-yield passengers. And there's a war going on, there's no doubt about it."

Last fall, American Airlines announced it was upgrading its premium-class seats on international flights, an effort to keep it competitive with other U.S. carriers and international competitors such as British Airways and Virgin Atlantic.

"They are all trying to respond to what the business traveler wants," Mitchell said.

At 6 feet 5, he said he knows firsthand how a poorly designed seat can make it impossible to get any work done on a flight.

"I want to be able to work on my laptop, even if someone in front of me reclines," he said. It also has to be comfortable enough to sleep in."