WASHINGTON, September 2, 2008 (AFP) - Teen suicides in the United States jumped sharply in 2004 and 2005, researchers said Tuesday, with the disturbing increase sparking confusion and concern among doctors and parents.

The increase came after more than a decade of decline, and when researchers first took notice of the shift upward in 2004, they thought it might be an anomaly.

But numbers for 2005, the most recent available data for teen suicides, while five percent lower than 2004 were still well above the long-term trend, according to Jeff Bridge, of Nationwide Children's Hospital in Columbus, Ohio.

'Suddenly in 2004 we see the sharpest increase in the past 15 years and it appears that it's persisting into 2005,' Bridge said in a summary of the research findings.

The scientists were at first surprised by the 2004 data, which showed about 326 more suicides among youth under 20 years old than would have been expected by the trend of the previous eight years.

The rate in 2004 was a high 4.74 per 100,000 people.

At first many thought it was a statistical aberration, but then the figures for 2005 showed about 292 more teen suicides than expected. The rate was an improved 4.49 per 100,000, but still significantly higher than the trendline.

'The fact that this significant increase in pediatric suicides continued into 2005 implies that the alarming spike witnessed from 2003-2004 was more than just a single-year anomaly,' said Bridge, who led the study that produced the figures.

Explanations for the rise are not readily available. One factor could be the influence of social networking on the Internet, which took off around that period.

Other possibilities were increased suicides among US soldiers -- some are only in their teens -- and also the effect of so-called new 'black-box' warnings placed on antidepressant drugs.

Noting a 20 percent decline in the drugs' use, researchers think that the warnings may possibly have deterred suicidal youth from getting access to the medications, though Nationwide Children's Hospital's John Campo warned that a direct connection to the spike in suicides had not been established.

The research was published Tuesday in JAMA, the Journal of the American Medical Society.