GUATEMALA CITY, Feb 19 (Reuters) - Drug smugglers are using sophisticated devices like satellite positioning systems to outwit police and move more South American cocaine by sea, a senior U.S. official said on Thursday.

Traffickers who used to fill speedboats with tanks of fuel to power long, clandestine sea journeys, leaving less room for drugs, are now fitting them with Global Positioning Systems so they can meet up with refueling ships at sea.

Using GPS devices to hook up with a waiting ship loaded with fuel means a much bigger stash of drugs can be packed on the boats, said Perry Holloway, director of anti-drug operations at the U.S. Embassy in Colombia.

"It's one of the main tools for drug traffickers because before they needed to pack much more fuel than cocaine but now they can calculate exactly," Holloway, in Guatemala for talks with local officials on fighting drug cartels, told a news conference.

A large chunk of the roughly 1,000 tonnes of cocaine grown and processed annually in Colombia, Bolivia and Peru is believed to pass though Central America, from where it is moved north to U.S. consumers overland or in small boats or planes.

Guatemala's government is grappling with an influx of Mexican and Colombian drug gangs operating in the country.

Drug cartels opting for sea routes use an array of innovative methods such as hiding drugs in secret compartments in fishing boats beneath huge stacks of nets and catch, or sailing hundreds of miles off course to fool authorities.

More recently, cartels have begun using homemade submarines that float invisibly just below the surface of the water.

"It's a new phenomenon. We are catching more semisubmersibles. They appear ten centimeters above the water and they're very difficult to find," Holloway said.

The subs are also equipped with GPS units and use the same mid-ocean refueling methods as the small boats, allowing them to travel from Colombia to the United States nonstop. (Reporting by Sarah Grainger; editing by Mohammad Zargham)