American Airlines and Delta Air Lines increased fares in most US domestic markets in the face of higher jet fuel prices, the airlines said on Thursday.

But the increase was not matched by US Airways and Continental Airlines, while United Airlines and Northwest Airlines said they were studying it.

Typically, airlines tend to roll back a fare increase if it is not matched by competitors.

"The challenge in this environment is getting competitors to join in," said Neil Bainton, chief operating officer of airline travel planning service FareCompare. "The fact that Delta jumped in means that at least there is some momentum."

If this increase sticks, it would be the 10th such broad-based increase out of 16 attempts this year, according to J.P. Morgan Securities analyst Jamie Baker.

American raised one-way fares by USD$5 to USD$20 on Wednesday night, with the extent of the increase depending on the length of the flight, spokesman Tim Wagner said. Delta followed on Thursday with a USD$10 one way increase, spokeswoman Betsy Talton said.

"Jet fuel spot price is about 15 percent higher than this time last year," Wagner said. "And fares are still as low or lower on average than they were in 2000 and before."

American said the increase does not affect sale fares that it has in place for the holiday travel season.

American raised fares by USD$5 for flights up to 500 miles, by USD$10 for 501 miles to 1,000 miles, by USD$15 for 1,001 miles to 2,000 miles, and by USD$20 for more than 2,000 miles.