Protesters in Honduras have burned down government offices and demanded that US drug enforcers leave the area following the fatal shooting of four people.
Two men and two pregnant women were shot dead in a boat on the Patuca River in north-eastern Honduras on Friday, local officials say.
US and Honduran officials say police only fired shots after the helicopter they were in was fired at by smugglers.
The local mayor says they were innocent fishermen caught in the crossfire.
The US Drug Enforcement Administration (DEA) has confirmed that some of its agents were on a US-owned helicopter with Honduran police officers when the shooting happened.
The aircraft was chasing a small boat suspected of carrying drugs on the river, they told the Associated Press news agency.
Ricardo Ramirez, chief of Honduras' national police force, said the operation "was carried out with the support of the DEA" and that an assault rifle was seized at the scene.
DEA officials confirmed that their agents were aboard the helicopter.
"We were there in a support role, working with our counterparts," DEA spokeswoman Dawn Dearden told AP.
US Embassy official Matthias Mitman in Tegucigalpa said in a statement that "the US assisted Honduran forces with logistical support in this operation".
According to the New York Times, US helicopters were scrambled to help seize a boat carrying cocaine. As they did so, a second boat arrived and opened fire.
A US official speaking on condition of anonymity told AP that said several helicopters owned by the US State Department were involved in the mission and carried members of Honduras' National Police Tactical Response Team.
They said the aircraft were piloted by Guatemalan military officers and outside contractors.
Lucio Baquedano, mayor of the coastal town of Ahuas, said police fired on the wrong boat.
"These innocent residents were not involved in the drug problem, were in their boat going about their daily fishing activities... when they gunned them down from the air," he told AP.
He said people had vented their anger at the government offices in Gracias a Dios region because they "sought revenge" against the government.
'Persona non grata'
The leaders of several of the ethnic groups in the area said in a joint statement that "the people in that canoe were fishermen, not drug traffickers.
"For centuries we have been a peaceful people who live in harmony with nature, but today we declared these Americans to be persona non grata in our territory."
The US says 79% of all cocaine smuggling flights leaving South America land in Honduras first.
According to the US State Department, DEA officials based in the country helped seize more than 22 tonnes of cocaine last year - nearly four times more than in 2010.
The US has personnel stationed at Soto Cano Air Base, and military equipment from the base has been used in drug operations before.
But US Embassy officials say that neither troops nor equipment from the base were involved in Friday's incident.