They were abducted in Arauca state in northeastern Colombia on 25 February after visiting an indigenous people, the U'wa, whom they were trying to protect.

Victim Terence Freitas was found blindfolded

The US State Department described the killings as "cold blooded murder" and called on the Colombian Government to capture and extradite those responsible.

Venezuelan authorities said the three bullet-ridden corpses - two women and one man - were blindfolded and had their hands tied behind their backs.

Gunfire

The bodies were discovered on Thursday about 100 feet (30 metres) from the river Arauca which separates the two countries in southeastern Venezuela.

Ms Washinawatok was a Menominee Indian

"Everything indicates that they were killed on the Colombian side and thrown over here," Interior Minister Luis Miquilena told Reuters.

Reports named the victims as Terence Freitas, 24, an environmental scientist from California, Ingrid Washinawatok, 41, of New York City and Laheenae Gay, 39, of Hawaii.

Reuters said their bodies were located after an army patrol was drawn to the scene by bursts of heavy gunfire.

"They had just been killed," said regional army commander General Rigoberto Martinez.

Rebels blamed

The three Americans were members of an international campaign trying to force a US multinational to abandon plans to drill for oil near the ancestral homelands of Colombia's U'wa Indians.

Colombian police and a U'wa spokesman blamed the attack on Colombia's largest guerrilla group, the Revolutionary Armed Forces of Colombia (FARC).

"We condemn the FARC in the strongest possible terms for this barbaric terrorist act," a US State Department spokesman said.