The release of hostages held by Marxist guerrillas in Colombia will not take place on Friday due to security concerns, a Red Cross official said.

"The second phase of the operation (picking up the hostages in a secret location) cannot be done today," said Barbara Hintermann of the International Committee of the Red Cross.

"The ICCR does not carry out nighttime operations for security reasons," she told a news conference in Bogota.

Venezuelan President Hugo Chavez had said the first rescue helicopters would head to Colombia on Friday about 3:00 pm (1930 GMT) to begin the operation.