CHICAGO — United Airlines plans to ground as many as 20 airplanes or 4 percent of its fleet and further cut capacity in 2008 to soften the blow of soaring oil prices that could add $1 billion to its fuel tab over last year.

The nation's second-largest carrier said today it will ground and sell back to lessors 15 to 20 older, narrow-body 737s that are less fuel-efficient than others in its 460-plane fleet. It did not immediately specify what domestic flights or routes could be trimmed.

UAL Corp.'s United, like other airlines, has grown more aggressive in passing on higher fuel costs to customers. It raised ticket prices by as much as $50 per round trip last week — an increase matched by other carriers. Airlines are also adding fees for some offerings including, at United, a $25 fee for checking a second bag.

It remains to be seen whether that will be enough to stem the fuel cost setback without cutting jobs. Delta on Tuesday said it will offer voluntary severance payouts to roughly 30,000 employees, or more than half its workforce, and will cut domestic capacity another 5 percent because of skyrocketing fuel prices.

Fuel is United's largest expense, and every $1 increase in the price of a barrel of oil boosts its costs by about $60 million. Oil prices have shot up by about $20 barrel in the last six weeks, raising United's projected 2008 costs by some $1.2 billion.

"We are taking a prudent step now by reducing our fleet, taking assets out of the network that don't make sense at these fuel prices, to better position United to be successful in an ever-challenging environment," Chief Financial Officer Jake Brace said. He outlined the changes at a conference for airline industry investors in New York.

United also is looking for other places to cut costs and said it has increased its fuel hedges since January, with 20 percent of its fuel hedged for 2008.

CEO Glenn Tilton defended the need to pass on higher costs to customers in a message to employees, noting that the economic environment has changed significantly over the past several weeks.

"Continued uncertainty about the overall U.S. economy with the price of fuel at historically high levels has put significant pressure on all U.S. carriers," he said. "U.S. airlines must function like other businesses, making investments and providing service where we can do so profitably."

United's management remains interested in consolidation, but the run-up in fuel prices appears to have chilled that talk for now.

The company plans to announce specifics of any capacity cuts when it reports first-quarter results in late April or early May.

United shares rose 4.8 percent, or $1.01, to $21.92 in afternoon trading.