UPDATE 1-US FDA: Arthritis drugs raise cancer risk in kids

* TNF blocker drugs to carry stronger warnings

* Cancer risk occurs about 30 months after treatment

* FDA says agency, companies working to understand risk

WASHINGTON, Aug 4 - Prescription drugs used to treat rheumatoid arthritis and other conditions can increase the risk of potentially deadly cancer in children and teenagers, U.S. health regulators said on Tuesday in ordering stronger warnings on such medications.

The Food and Drug Administration, which urged greater caution with use of the so-called TNF blockers last September, said an analysis of reported cancer cases in children using the drugs, "showed an increased risk of cancer, occurring after 30 months of treatment on average."

Some of the reported cases were fatal, the agency added.

Anti-TNF drugs include Johnson&Johnson's Simponi or golimumab, as well as its Remicade or infliximab. They also include Abbott Laboratories Humira or adalimumab, UCB SA's Cimzia or certolizumab pegol, and Amgen Inc and Wyeth's Enbrel or etanercept.

The medications are used to treat other inflammatory conditions, including the bowel disorder known as Crohn's disease.

The FDA, in a statement on its website, said its analysis of the increased cancer risk in children showed about half the cases involved lymphoma, a cancer that targets the immune system.