By Jeremy McDermott BBC correspondent in Colombia

The Colombian guerrilla group holding seven foreign tourists hostage has said it is prepared to exchange them for rebels held in prison.

The ELN, or National Liberation Army, has made its first statement on the conditions for the release of the tourists - a Briton, Mark Henderson from North Yorkshire, a German, a Spaniard and four Israelis.

The group was kidnapped as they hiked in the mountains of the Sierra Nevada more than three weeks ago.

Another Briton seized at the time managed to escape and make his way to safety.

Pablo Beltran, one of the ELN's top commanders, has made the rebel group's first statement about terms for the possible release of the foreign hostages.

HOSTAGES HELD Briton: Mark Henderson German: Reinhilt Weigel Israelis: Beni Daniel, Ortaz Ohayon, Ido Joseph Guy, Erez Altawil Spaniard: Asier Huegun

He said the seven were being investigated to ensure they were not intelligence operatives or military spies but that he would be prepared to release them now in exchange for rebels held in prison.

The idea of a prisoner exchange is nothing new.

A larger rebel group, the FARC, have been demanding such an exchange for years and are currently holding more than 50 political hostages, including three kidnapped Americans.

They have said they will not be released until an exchange agreement has been reached and some of their hostages have spent more than five years in captivity.

The hope is that the ELN will not copy the FARC.

Up until now they have only kidnapped for ransom.

Three Catholic priests are making their way up to the mountains of the Sierra Nevada to meet ELN guerrillas and visit the kidnapped tourists.

The priests are hoping to persuade the rebels to release the hostages without any conditions.