The long decline in sexual activity among U.S. teenagers, hailed as one of the nation's most important social and public health successes, appears to have stalled.

After decreasing steadily and significantly for more than a decade, the percentage of teenagers having intercourse began to plateau in 2001 and has failed to budge since then, despite the intensified focus in recent years on encouraging sexual abstinence, according to new analyses of data from a large federal survey.

The halt in the downward trend coincided with an increase in federal spending on programs focused exclusively on encouraging sexual abstinence until marriage, several experts noted. Congress is currently debating funding for such efforts, which receive about $175 million a year in federal money and have come under fire from some quarters for being ineffective.

The leveling off in teen sexual activity is worrying experts and advocates across the ideological spectrum. The fall of such activity has been one of the key forces behind a historic drop in teen pregnancy rates and has bolstered efforts to protect teenagers from sexually transmitted diseases such as AIDS.

"It is alarming," said Susan Philliber of Philliber Research Associates, a private firm that studies teen sexuality. "We've had such a wonderful decade's run of getting the rate down. For it to level out causes everyone to go, 'Uh-oh.' "

Experts are unsure of the reasons for the change, but they speculated that it could be the result of a combination of factors, including growing complacency among the young about AIDS and the possibility that some irreducible portion of the teenage population can never be dissuaded from having sex.

"At a certain point, it becomes really hard to change basic human behaviors," said John Santelli, who studies teenagers at Columbia University. "I think what we're seeing is the limits of the emphasis on abstinence as the primary message."

But abstinence proponents argue that, if anything, the data underscore the need for greater emphasis on encouraging youngsters to abstain from sex until marriage.

"We need to increase abstinence education and give more dollars to abstinence education. It is the healthiest program we have for young people," said Leslee Unruh of the National Abstinence Clearinghouse.

A recent study of four separate abstinence programs, conducted for the Department of Health and Human Services by Mathematica Policy Research, a nonpartisan firm, found no evidence that the programs delayed the start of sexual activity among teens, but Unruh and others said such programs need more time and wider use to counter pervasive messages encouraging teens to have sex.

"Teenagers today live in an MTV-driven culture and are bombarded by sexual messages that say it is normative for them to get involved sexually," said Charmaine Yoest of the Family Research Council. "We need a message that sexual experimentation as a teenager is unhealthy."

The House last week approved a $28 million increase in spending on abstinence programs -- Democratic leaders said it was intended to win Republican support for the annual health and education funding bill -- but the Senate is considering a $28 million cut, largely because of concerns about the programs' efficacy.