By Eder Lopez

SALINA CRUZ, Mexico, July 18 (Reuters) - Mexico found nearly six tonnes of cocaine in a makeshift submarine seized this week off the Pacific coast, the navy said on Friday.

The 32-foot (10-metre)-long, green fiberglass craft was designed to travel just beneath the water, leaving almost no wake.

It was one of Mexico's largest maritime drug seizures and the first time the country has seen drug smugglers using a submarine, the navy said.

Four Colombians aboard the submarine said they had navigated up the Pacific coast from Colombia, according to the navy.

Colombian officials told Reuters last month that diesel-powered drug submarines travel up to two weeks to reach Central America and Mexico. The drugs are then hauled overland into the United States.

Mexican special forces raided the submarine on Wednesday after they spotted it from the air by helicopter. They detained the crew and brought them and the vessel back to the Pacific port of Salina Cruz in Oaxaca state.

"This is going to force us to increase surveillance," Vice Adm. Jose Maria Ortegon told reporters in Salina Cruz.

Colombia's navy says each homemade submarine costs about $600,000 to make and is outfitted with propellers.

The submarine find comes as troops on Thursday discovered 12 tonnes of marijuana in trucks near Tijuana at Mexico's border with California, police said.

President Felipe Calderon's government has made several huge drug seizures after deploying thousands of troops to trafficking hot spots since taking office in December 2006.

But drug trade specialists in Mexico say troops and police are failing to tackle drug gangs' financial networks and go after money launderers, which they say would do more to weaken the cartels.

Some 1,700 people have been killed in drug gang violence in Mexico so far this year. (Additional reporting by Lizbeth Diaz in Tijuana; Editing by Xavier Briand)

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