BY GONZALO GUILLEN

Colombian leftist rebels are trying to purchase anti-aircraft missiles on the black market, as they take heavy losses from the Colombian Air Force in a war they suddenly seem to be losing.

According to a source within the Colombian judiciary that had access to files found on the laptop computer of a recently fallen FARC guerrilla leader known as ''Iván Ríos,'' the rebel sent an e-mail to supreme commander Manuel ''Sureshot'' Marulanda saying that anti-aircraft weapons were "an urgent necessity.''

During a telephone interview with El Nuevo Herald, the source read fragments of a financial and tactical report sent by Ríos to Marulanda which reveals the rebels are more isolated, demoralized and financially strained than the Colombian military had anticipated.

Files found in computers belonging to another killed FARC leader, a man known as Raúl Reyes, also suggest the rebels are looking to obtain anti-aircraft missiles.

According to intelligence reports obtained by El Nuevo Herald, the FARC may already have a few U.S.-made Stinger surface-to-air missiles or possibly the Soviet version, SA-14 Gremlin and SA-16 GIMLET.

Citing U.S. intelligence and independent investigations, the report says that "There are strong indications that the FARC have these weapons at their disposal and that they have not been used because they are being reserved for a strategic operation.''

The report suggests that primary targets for the use of these weapons could be Colombian President Alvaro Uribe's plane, surveillance and recognizance aircraft and bombers and fighters that have recently proven very effective in strikes against FARC jungle camps.

Up to now the only anti-aircraft weaponry found in possession of the FARC have been 50 caliber machine guns.

In the past, the FARC has ineffectively used Russian rocket-propelled grenades (RPG-7V) or home-made mortars as surface-to-air projectiles.

The e-mail sent by Ríos to Marulanda also contains six pages of answers to 19 questions regarding the financial and tactical position of two guerrilla fronts under Ríos' command.

Ríos reports that recruitment, 'has gone down in both fronts, they have lost their dynamic, there is no accounting for the losses and desertions. The main reason is the forced displacement of the 'campesinos' [farmers displaced by paramilitary groups and the Colombian authorities] from the areas where we have been acting and recruiting.''

In another file found in Ríos' computer, there is a report on the execution of 200 of his own guerrillas that he ordered believing them to be "infiltrated enemies.''