PARIS, April 1, 2008 (AFP) - Colombia must stop all military operations that endanger hostages held by rebels, an international human rights group and support groups for French-Colombian hostage Ingrid Betancourt said Tuesday.

President Alvaro Uribe must 'order an immediate end to all the military operations that threaten the lives of people held hostage by the FARC (Revolutionary Armed Forces of Colombia),' wrote the International Federation for Human Rights (FIDH) and the support groups in an open letter to Uribe.

'It is with immense concern that we have noted that military operations and bombing continue and are intensifying in the regions where it is presumed that the FARC hold people in captivity,' they wrote.

The Marxist FARC, which has been fighting the Colombian government for more than 40 years, is believed to be holding more than 700 people hostage in the jungles of the Latin American state.

Betancourt is among 39 high-profile hostages, including three US defence contractors, whom the FARC wants to exchange for 500 rebels held in prison.

The FIDH said in its letter Tuesday that if Betancourt or others died because of Colombian army attacks, 'you (President Uribe) will no longer be the democratic leader you aspire to be but the torturer of these defenceless people.'

A report said Monday that Betancourt, 46, who is believed to be gravely ill after her six years in captivity, was refusing food or medicine offered by the rebels and urgently needs a blood transfusion.

Concern about her health grew in recent days after reports that the FARC took her to medical facilities in late February, prompting France to put a plane and a medical team on standby in case she is freed.