A twin-engine turboprop plane that crashed in northern Colombia while on a US counter-drug mission, killing three American contractors and a Panamanian aboard, had been tracking a suspected smuggling vessel over the western Caribbean when it lost radio contact, the US military has said. Two other Americans aboard the Dash 8 were seriously injured in the crash, which happened before dawn on Saturday.

Under contract with the US air force, the plane was a "prospector" aircraft, equipped with surveillance equipment and employed to track speedboats and other vessels that smuggle cocaine north from Colombia, said Jody Draves, spokeswoman for the US-sponsored multinational task force in Key West, Florida, known as JIATF-South, that runs interdiction in region. Draves said the plane was operating in co-ordination with a Colombian surface vessel and had flown out of Panama. It was near that country's border that it crashed, near the city of Capurgana.

The two injured Americans were rescued by Colombian soldiers and taken to a hospital in the capital, Bogota, Southcom said in a statement. The names of the Americans were withheld, pending notification of next-of-kin. Panama's national air service identified the deceased Panamanian guardsman as Lieutenant Lloyd Nunez. Host-country nation service personnel routinely accompany US military contractors on such missions.

"We express our sympathies to the families of the deceased, and are particularly saddened by the loss of a Panamanian air national guardsman," said General John Kelly, commander of US southern command. "We also want to thank the Colombians for their outstanding rescue and recovery efforts."

General Nicasio de Jesus Martinez, commander of the Colombian army's fourth brigade, whose troops traveled to the accident scene, ruled out the possibility that the plane was shot down by rebels active in Colombia.

"There was no aggression, no impact," said Martinez, adding that it was too soon to know if the crash was caused by mechanical failure, human error or the weather. Southcom also said there was no indication the plane was shot down.

The region where it crashed is mountainous jungle and rebels of the Revolutionary Armed Forces of Colombia, or Farc, operate there along with drug traffickers. Local farmers reported that the plane went down at about 1am in a rural part of the municipality of Acandi, said mayor Gabriel Jose Olivares. Capurgana is in the municipality of Acandi.

Carlos Ivan Marquez, chief of Colombia's national office for disaster response, said the surviving Americans had injuries including multiple bone fractures and burns over at least 40% of their bodies.

Santiago Castro, director of Colombia's civil aviation agency, said the plane was not civilian so he could not provide details about its route, origin or destination. The plane was contracted to provide detection and monitoring of drug trafficking routes in the coastal region of Central America as part of Operation Martillo, Southcom said.

Operation Martillo (Hammer) is part of the $165m, US-led regional security initiative that focuses on the seas off Central America, key shipping routes for 90% of the cocaine headed to the US. Fourteen countries participate: Belize, Canada, Colombia, Costa Rica, El Salvador, France, Guatemala, Honduras, the Netherlands, Nicaragua, Panama, Spain, the United Kingdom and the US. Chile has also contributed to the operation.