The US military last sent any significant aid and equipment into Guatemala 50 years ago, establishing a base to support counter-insurgency efforts during a guerrilla uprising. That movement led to 36 years of war that left 200,000 dead, mostly indigenous Maya farmers. Washington pulled out in 1978.

Guatemala signed a treaty on 16 July allowing the US military to conduct the operations. Less than a month later, a transport plane flew into Guatemala City from North Carolina loaded with the marines and four UH-1 “Huey” helicopters.

This week, the marines have been patrolling waterways and the coastline, looking for fast power boats and self-propelled “narco-subs” used to smuggle drugs along Central America’s Pacific Coast. The subs can carry up to 11 tons of illegal cargo up to 5,000 miles.

General Douglas Fraser, head of the US military’s Southern Command, said the area was a top transit spot for South American cocaine destined for the United States.

“Key arrival points in Central America are the north-east coast of Honduras and then the Pacific coast of Guatemala,” Gen Fraser said. “We changed our strategy to be more persistent … to see if we could have a bigger impact on trafficking organisations.”

Washington estimates that 90 per cent of South American cocaine heading to the US passes through Central America and attributes a rise in violent crime in the region to transnational drug smugglers.

Trafficking routes have shifted in recent years as governments crack down on smugglers, forcing security teams to adapt.

Eight out of every ten tons of cocaine that reaches the US are loaded on vessels known as “go fasts” – open-hulled boats 20 to 50 feet long with as many as four engines, according to the US defence department.

Gen Fraser arrived in Guatemala on Tuesday and held a meeting with the country’s president, Otto Perez.

Mr Perez, a retired general who served during the country’s brutal civil war, took office in January, promising a crackdown on violent crime in the tiny Central American nation that has been plagued by bloody street gangs and powerful drug cartels.

Mr Perez has dispatched special Guatemalan forces to the country’s porous and sparsely populated northern border with Mexico and promised to boost the army by 2,500 soldiers during his four-year term.

If the US marines find suspected boats, they will contact their Guatemalan counterparts in a special operations navy unit that will move in for the bust. The marines will not go along on arrest missions, but they do have the right to defend themselves if fired on.