UPDATE 3-U.S. orders warning on Pfizer, Glaxo smoking drugs

* FDA: Chantix, Zyban need to warn of mental problems

* Agency orders further clinical trials

* Pfizer says Chantix benefits outweigh risks

* Chantix alert on accidents made more prominent (Adds further reaction, edits)

By Susan Heavey

WASHINGTON, July 1 (Reuters) - U.S. health officials on Wednesday ordered Pfizer Inc (PFE.N) and GlaxoSmithKline PLC (GSK.L) (GSK.N) to add strong "black box" warnings on their anti-smoking drugs to highlight the risk of serious mental health problems, including suicidal thoughts.

The warnings, which must be added to Pfizer's Chantix and Glaxo's Zyban, follow more than five thousand reports of depression, hostility and other behavioral changes, the Food and Drug Administration said.

FDA officials said there was still a need for the medications, their concerns tempered by smoking's health dangers, such as lung cancer and heart disease.

"We don't want to scare people off from trying to use the medication to stop smoking. We just want them carefully monitored," said Dr. Curt Rosebraugh, who oversees the FDA office that reviews smoking-cessation drugs.