Colombian President Alvaro Uribe has said an American and a Colombian were shot in cold blood, after a light plane came down in an area controlled by left-wing FARC rebels.

The four Americans and one Colombian were all reportedly alive when their single-engine Cessna came down in the jungles of Caqueta province.

FARC rebels reached the survivors before local army units could find the plane, and killed the two "execution-style, in cold blood", according to General Jorge Enrique Mora, Colombia's senior military commander.

It is the first time during the four-decade Colombian civil war that an American working for the US Government has been killed by rebels.

The other three Americans have been taken hostage.

"We demand that the crew members be released unharmed immediately," said a US State Department spokesman.

US officials said that the men were "civilian specialist contractors", but the BBC correspondent in Bogota says they were believed to be working with the US Central Intelligence Agency (CIA).

Failed landing

The two bodies have now been taken to a nearby Colombian military base for identification.

A Colombian military source quoted by Reuters news agency said that one man had a bullet wound in the head, and the other had been shot in the chest.

It was earlier reported that both the dead men were Americans.

The plane crashed near Florencia, Caqueta Province, 380 kilometres (235 miles) south of the capital Bogota, at 0900 (1400GMT) on Thursday.

It came down just minutes before a scheduled landing, Colombia's Civil Aviation agency said.

"We believe it had engine trouble and it was attempting to make an emergency landing in Florencia," a US State Department official in Washington said.

Rebel control

Colombia's rebel groups have a long history of abducting people for ransom.

While Colombian Government forces largely control the region's towns, rebels still hold sway over much of the countryside.

The US actively supports the Colombian Government's anti-drug operations in the coca fields around Florencia, as well as combating the rebels.

In July 1999, an American spy plane monitoring the skies and intercepting communications traffic crashed into a mountainside in Colombia, killing the seven men on board - five of them US military personnel.