By Patrick Markey

BUENAVENTURA, Colombia, July 4 (Reuters) - Colombian Marine Sgt. Edwin Garcia scans the walkways weaving through slums in the country's major port on the lookout for gangs battling in a turf war over cocaine routes into the Pacific Ocean.

Colombia's guerrilla war may have eased, but in Buenaventura's maze of rivers and wooden shacks, Marines are engaged in daily cat-and-mouse with rebels, militias and traffickers vying for control of drug smuggling from the jungle coastline.

Seven bombs and grenades exploded in the city's poor barrios in two days last month, wounding 23 people, and a day later a blast killed a man and a toddler on the outskirts of the port, which handles half of Colombia's shipments.

"These barrios are tough, next to the sea where they can get out drugs and arms," Garcia said among the dirt tracks of Punta del Este barrio. "They know how to move here, they flee into the sea or just go house to house."

Contrasting with slums, foreign merchant vessels float off the harbor and trucks stream to nearby cities. The port, which expects to handle 10 million tonnes of freight this year, carries much of Colombia's coffee and sugar exports.

But the city has suffered an uptick in violence with officials blaming urban militias from the country's largest rebel group, the Revolutionary Armed Forces of Colombia, or FARC, for a string of bomb attacks at the start of the year.

Recent attacks forced local authorities to impose a temporary curfew on the city.

With homicides in Buenaventura at least twice the national average and three out of four violent deaths linked to the armed gangs in barrios, Police commander Gen. Oscar Naranjo told Reuters the port remains one of his major challenges.

Buenaventura and Colombia's west coastline are now key routes for cocaine smuggling as rebels and drug barons muscle for control over a region with links to 180 foreign ports, analysts say.

Marines recently nabbed Edwin Torres, a suspected FARC militia leader, hiding under a roof in one of the city barrios.

"Today in Buenaventura there is no single armed group with full control of their territory, but sadly the state has not yet been able to fully establish full control either," Buenaventura Mayor Saulo Quinones said.

PORT AND POVERTY

Set partly on a densely populated island, Buenaventura is surrounded by wooden and tin-roof homes many on stilts over water, where the mostly Afro-Colombian residents struggle to survive on fishing, timber and limited commerce.

Residents say low investment, poverty and scare job opportunities can breed discontent. President Alvaro Uribe visited recently to promise more infrastructure funds and sign deals to extend and expand busy port concessions.

But a security crackdown is steadily yielding results and pushing out traffickers further north and south of city, says Marine Col. Hector Aguas of the Second Fluvial Brigade.

Authorities caught Olmes Duran, known as "Lord of the Ports" and the suspected head of a Buenaventura and Pacific coast cartel, and last week unearthed 6.5 tonnes of cocaine -- the latest large captured stash in the area.

Underscoring the region's importance, the military believes the recent wave of bombings was triggered by the killing of Milton "J.J." Sierra, a top FARC commander who authorities say was in charge of Pacific drug operations.

"The Pacific is one of the central zones ... which evidently drug traffickers have in their sights," Aguas said. "What we have is basically a dispute for territory."

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