The airline lounge competition is heating up around the globe, and two new entrants, American and United, have, somewhat belatedly, become serious contenders.

Entry usually requires the purchase of a business- or first-class ticket, and some V.I.P. lounges charge thousands of dollars per visit by individuals or small groups of travelers. There are some exceptions for holders of certain credit cards, and occasionally a traveler can make a one-time visit for a relatively low fee.

Many carriers use their top-tier lounges as “an extension of their premium onboard experience,” promoting them on their websites, said Henry Harteveldt, a travel industry analyst and president of Atmosphere Research Group.

One challenge facing any airline, alliance or company aiming to expand existing lounges or open new ones could be space constraints, Mr. Harteveldt said, noting, “There is a limited amount of real estate, especially in older terminals, when the lounge concept was different.”

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