The Americans' plane had crash-landed in the jungle

In a statement released on Monday, the Revolutionary Armed Forces of Colombia (FARC) demanded a demilitarised zone from the government.

"The three gringo prisoners of war in the custody of our organisation will be liberated along with other Colombian prisoners of war once an exchange materialises in a large demilitarised zone," the statement said.

The group also demanded the release of militants held in Colombian militants.

FARC accuses the three men of being CIA agents.

Washington has denied the men were CIA agents, and said they were contractors for the defence department.

However, the US has not identified the men nor stated what mission they were on.

Fierce fighting has taken place as the Colombian army tries to close in on the guerrilla group.

US President George Bush is sending 150 extra troops to Colombia to help the search operation.

The FARC said in its statement the decision, saying it was an "invasion by the United States of our country."

Reward offered

There are several hundred US military personnel in Colombia, including some special forces, but they are not allowed to take part in combat.

The US has spent $2bn in recent years to help Colombia tackle its illegal drugs trade, and recently lifted restrictions stopping the use of that aid against the guerrillas.

The three Americans held by FARC vanished when their plane crash-landed on 13 February in the remote southern province of Caqueta, long a rebel stronghold.

A fourth American and a Colombian army sergeant who were also aboard the US Government Cessna plane were found shot dead at the jungle site.

This is the first time that US Government employees have been captured during Colombia's four decades of civil war.