Dr Lester Crawford, acting commissioner for the Food and Drug Administration (FDA), said the agency sought to balance the increased risk of suicidal tendencies against the known benefit of treating depression in children.

"We continue to believe, however, that these drugs provide significant benefits for paediatric patients when used appropriately," he said at a news conference.

The FDA announcement comes one month after public hearings at which medical experts urged the agency to reclassify the drugs. On average, 2-3% of children taking antidepressants have increased suicidal thoughts, independent experts found.

In 24 trials involving more than 4,400 patients taking antidepressants, researchers found a greater risk of increased suicidal thoughts and behaviour during the first few months of treatment. Celexa, Prozac and Zoloft posed lower risks for children, while Luvox, Effexor and Paxil had higher risks of increased suicidal thoughts and behaviour.

Prozac is the only antidepressant approved by the FDA for use for treating depression in paediatric patients. Anafranil, Prozac, Luvox and Zoloft have been used for treating obsessive compulsive disorder in paediatric patients.

The FDA said in a statement that it recognises that depression "can have significant consequences in paediatric patients if not appropriately treated. The new warning language recognises this need but advises close monitoring of patients as a way of managing the risk of suicidality."

Because the warning - the highest available - is primarily aimed at doctors, the FDA is also creating a medication guide for patients to advise them of the risk.

Parents will be advised to look for warning signs in children that include worsening depression, agitation, irritability, and unusual changes in behaviour.

To make sure the medication guide is available with each prescription or repeat, Dr Crawford said the FDA would work with manufacturers to develop "unit of sale" packaging for all antidepressants.