WASHINGTON (Reuters) - U.S. drug reviewers on Tuesday said Arena Pharmaceuticals Inc's obesity pill appeared to help people lose weight and was unlikely to cause tumors in humans, but questioned if the company had provided enough data to rule out heart problems.

Lorcaserin is one of three new potential obesity treatments vying to gain U.S. approval and be the first new weight-loss treatment on the market in over a decade, after initial rejections over safety issues.

Obesity, a leading cause of diabetes, heart disease and other serious health problems, has reached epidemic proportions in the United States with nearly a third of the population falling into the category.

But the FDA has set a very high approval bar for weight loss drugs because such a large portion of the general population is likely to want to take them.

The FDA first rejected lorcaserin, which is being developed in partnership with Japanese drugmaker Eisai Co Ltd, in October 2010, citing potential cancer risks.

Arena resubmitted its application with more data to show that the previous findings of tumors in rats did not apply to people, which seemed to assuage some of the FDA's concerns.

Shares of Arena shot up nearly 18 percent to $3.20 in morning trading on Nasdaq after the FDA posted its review online.

The FDA documents come ahead of an advisory panel of outside experts, which will review the pill on Thursday. The FDA will make its final decision by June 27, taking into account the panel's recommendations.

The FDA's main concern seemed to be risks to heart valves seen with lorcaserin, which could be a sign of broader heart damage. Heart problems have dogged the obesity field for decades, making the FDA especially cautious about any potential risks.

The infamous diet drug fen-phen was pulled from the market in 1997 after reports of sometimes fatal heart-valve problems, and another diet pill, Meridia, was withdrawn in 2010 after being linked to heart problems.

The same FDA advisory panel that is due to review lorcaserin recommended in March that all new obesity drugs need heart safety studies, even if clinical trials do not initially show evidence of increased heart risk.

Analysts have said Arena's pill may have the lowest chance of approval, with a smaller weight loss compared to drugs from Vivus Inc and Orexigen Therapeutics Inc, meaning the FDA may be willing to accept fewer risks.

(Editing by John Wallace and Maureen Bavdek)