(Reuters) - U.S. Border Patrol agents and Mexican drug traffickers fought a gun battle across the Rio Grande river in south Texas, authorities said on Friday, the latest of a spate of cross-border shootings in recent months.

The Border Patrol said gunfire erupted on Wednesday after agents confronted a group of smugglers loading bundles of marijuana into two vehicles on the banks of the Rio Grande west of Roma, Texas, a town about 250 miles south of San Antonio.

The agents opened fire after smugglers fleeing in a vehicle attempted to run them over. Armed traffickers on the Mexican side of the river then shot at the agents, who returned fire into Mexico, the Border Patrol said in a statement.

"Our agents had a posed threat," Rosalinda Huey, a spokeswoman with the Border Patrol's Rio Grande sector told Reuters. "They're trained to deal with that situation," she added.

No agents were injured by the gunfire and it is unclear whether any smugglers in Mexico were struck by bullets, she said.

Agents subsequently recovered nearly two tons of marijuana, with a value of more than $3 million. No arrests were made and the incident remains under investigation.

Drug traffickers in Mexico's northern Tamaulipas state frequently use rafts and ropes to haul marijuana over the Rio Grande to Texas, often under the protection of gunmen.

Huey said traffickers opening fire on agents was "just another tactic" as they sought to move drugs across the U.S. border, where additional agents, equipment and infrastructure have contributed to tightening security in recent years.

"Obviously, they've gotten more desperate," she said. "They're going to use more tactics to avoid apprehension or seizure of their narcotics."

The stretch of the Rio Grande - which is known as the Rio Bravo in Mexico - has recorded one other shooting incident involving Border Patrol agents since October 2011, she said. No injuries were reported.

Concern runs high among U.S. politicians over so-called "spill over" violence from Mexico, where about 50,000 people have been killed in raging drug violence since President Felipe Calderon launched an army-backed offensive against the powerful drug cartels after taking office in late 2006.

In response to a pattern of violence on the border river, Texas earlier this week unveiled the second of six new 'interceptor' gunboats to patrol the waterway. They are similar to Navy swift boats that plied the rivers of Vietnam during the Vietnam War.

A tally of recent cross-border shooting incidents in Texas include an exchange of shots last year between U.S. law enforcement officials and suspected drug runners near the south Texas town of Abram, according to news reports. In a separate incident, a West Texas road crew in Hudspeth County, east of El Paso, also came under fire from Mexico.

And in September 2010, U.S. citizen David Hartley was fatally shot while riding a personal watercraft on Falcon Lake, which straddles the Texas-Mexico border.

(Editing by Tim Gaynor)

[Image via Shutterstock.com.]

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