GUATEMALA CITY (Reuters) - U.S. and Guatemalan authorities captured a makeshift submarine loaded with an estimated 10 tonnes of cocaine in the Pacific Ocean off Guatemala, Guatemalan police said on Thursday.

Four men aboard the vessel -- three Colombians and a Mexican -- were arrested when it was detained Wednesday night by U.S. anti-drug agents and the Guatemalan Coast Guard some 175 miles off Guatemala’s Pacific Coast.

If the size of the seizure is confirmed, it would be the largest ever drug bust in Guatemala, which is becoming an important transit point for illegal drugs moved north by powerful Mexican trafficking cartels.

Drug gangs operate with impunity in the jungles of northern Guatemala, where they receive shipments of South American cocaine and ship them across the border into Mexico.

Mexico’s drug cartels have been moving into neighboring Guatemala as they seek to secure supply lines amid a brutal struggle for territory in Mexico that has claimed some 14,500 lives in the nearly three years since President Felipe Calderon came to power and launched an army crackdown.

Anti-drug patrols in the waters around Central America have turned up at least two other large submarine-like vessels that can be as much as 59-feet long and carry sophisticated navigation equipment. The steel-and-fiberglass vessels run partially submerged in an attempt to evade radar.