Leftist Colombian rebel Ricardo Palmera Pineda, known as "Simon Trinidad" and held in a United States prison, asked his comrades-in-arms to offer proof of life for the hostages being offered up for a swap, a mediator said Tuesday after meeting him.

Opposition Colombian Senator Piedad Cordoba, who is mediating in the process alongside Venezuelan President Hugo Chavez, said Simon Trinidad handed her a letter with the demand that she is to forward to the Revolutionary Armed Forces of Colombia (FARC).

Talks are currently ongoing to secure the release of 45 hostages held by the Marxist FARC in exchange for the freedom of close to 500 imprisoned rebels, including Simon Trinidad and "Sonia" who were extradited to the United States.

Simon Trinidad, like Sonia, said weeks ago in a meeting with Cordoba that he was willing to be left out of the swap if that makes the process easier.

Through Cordoba, the rebel also asked that Colombian President Alvaro Uribe be asked to reduce his demands on FARC.

The senator said that with the good disposition of Simon Trinidad and Sonia - whose real name is Anayibe Rojas - "an obstacle was brought down" for the process.

The meeting Tuesday at a Washington court building was also attended by two representatives of the Colombian Embassy in the United States, two FBI agents and a representative of the US State Department.

Cordoba said she will hand the letter from Simon Trinidad to Chavez, who is set to meet FARC leaders soon, though no date has yet been set for that meeting.