* New Galaxy S6 sports sleek metallic look
* Boasts battery and charging innovations
* Samsung shares hit near 9-mth high on mobile hopes
* Key launch in fight to stay No.1 vs Apple  

Reuters/Seoul

South Korea's Samsung Electronics Co Ltd unveiled its latest Galaxy S smartphones, featuring a slim body made from aircraft-grade metal, in a bid to reclaim the throne of undisputed global smartphone leader from Apple Inc.

Designed from scratch in an operation dubbed "Project Zero", the Galaxy S6 and its curved-edges variant are critical for Samsung's plans to reverse plunging smartphone revenues that led to its first annual earnings fall in three years in 2014.

A warm initial reception sent Samsung shares on Monday to their highest in nearly nine months. Some technology publications described the phones as Samsung's best-looking to date after its previous flagship device, the Galaxy S5, failed to impress.

The shares closed up 4.9 percent, outperforming a 0.6 percent rise for the broader market.

"We listen to our customers and we learn from our missteps," J.K. Shin, Samsung's mobile chief, told a packed hall in Barcelona ahead of the annual Mobile World Congress trade show, where the Galaxy S6 and the Galaxy S6 edge were unveiled on Sunday.

By some estimates, arch-rival Apple surpassed Samsung as the world's top smartphone maker in the fourth quarter, with record sales of the big-screen iPhone 6 and 6 Plus.

Samsung begins its fight-back on April 10 when the revamped Galaxy phones go on sale in 20 countries in one of the most important product launches in the company's history.

Stand-out features include a casing made from light-weight metal used in airplanes, a step up from the plastic that disappointed many critics of the S5, and Corning Inc's  Gorilla Glass on both front and back.

In addition to camera and screen upgrades, Samsung stripped out many of its unpopular in-house apps that infuriated users by gobbling up memory. The new phones will be powered by Samsung's own 64-bit, 14-nanometer Exynos processors, a departure from its previous reliance on Qualcomm Inc's chips.

In another departure, batteries in the new phones cannot be replaced. To compensate, the devices are the first from Samsung to support wireless charging without a dedicated accessory and can also get four hours of power with a 10-minute charge by cord.

Samsung is also touting the Galaxy S6's compatibility with a new mobile payments system it is preparing to launch in the United States and South Korea in the second half of this year.

The system, in partnership with major U.S. banks and credit card companies including Visa and MasterCard, will allow users to make mobile payments through magnetic strip-card readers without an external accessory needed for other models.

The rival Apple Pay system, launched in the United States in September and rapidly winning retailer support, requires merchants to install near-field card readers, limiting its reach as contactless credit cards have been slow to take off there.