Washington ? 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 a new analysis 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 speculate that it may be because 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.