By Jeremy McDermott BBC correspondent in Bogota

Paramilitaries are among the groups in talks with the government

US ambassador William Wood effectively torpedoed the Colombian government's proposals to effect a prisoner exchange with one of the groups.

He spoke out against the release of imprisoned guerrillas, and warned the US would enforce extradition warrants against them if they were released.

This puts the Colombian government in a very difficult negotiating position.

The government is now engaged in dialogue with three of the country's warring factions.

Condemned

However, the US, while professing to support all peace efforts, has again limited the government's negotiating options.

Mr Wood spoke out against the release of prisoners, just as the Colombian government is seeking to negotiate some sort of exchange involving jailed rebels for political hostages held by the guerrillas.

The ambassador has also condemned the leadership of the right-wing paramilitaries with whom the government is also holding peace talks.

He said that many of the paramilitary leaders are drugs traffickers and insisted that the US would enforce extradition warrants against them.

This would leave the Colombian government very little to offer in exchange for the paramilitaries laying down their weapons.