BOGOTA -- Tension mounted between Bogota and Caracas Sunday after Colombian paramilitaries accused the Venezuelan Army of military incursions in support of leftist rebels on Colombia soil, AFP reported.

Venezuela backed guerrillas from the Revolutionary Armed Forces of Colombia (FARC) in attacks on paramilitary forces in the Catatumbo region, 600 kilometers (360 miles) northeast of Bogota, the latter said.

"Colombian territory was attacked in late March and early April in combined actions by the FARC and sectors of the Venezuelan Air Force," the United Self-Defense Forces of Colombia (AUC) said in a letter posted on its website, addressed to Colombia's peace commissioner Luis Carlos Restrepo.

The FARC rebels crossed into Colombia from encampments in Venezuela, where they were receiving protection from Venezuela's national guard, the letter charged.

Colombia and Venezuela have a joint 2,220-kilometer (1,400-mile) frontier over which Colombian rebel groups make regular forays in both directions.

Most recently, an incident on March 21 in the La Gabarra region northeast of Bogota, near Tibu, the local population complained that Venezuelan aircraft including helicopters had entered Colombian air space.

They said the military hardware had bombarded an area where guerrillas and paramilitaries regularly clash.

Bogota has opened an investigation into the incident.

Rangel on March 29 launched accusations of his own over the "absolute impunity" of paramilitaries in Colombia.

Colombian Foreign Minister Carolina Barco on Saturday voiced concern over Rangel's assertions. Writing to her Venezuelan counterpart Roy Chaderton, in a letter published in Bogota's **El Tiempo** newspaper, Barco said: "The international community knows that the Colombian state fights against all violent groups, whether they be guerrillas, illegal paramilitaries or common criminals."

Venezuelan President Hugo Chavez and Colombian President Alvaro Uribe are due to meet on April 23 at Puerto Ordaz to refocus their cooperation in the fight against the leftist rebels and right-wing paramilitaries.

Relations between Venezuela and Colombia have deteriorated since Chavez's inauguration in 1998, with Bogota complaining at Caracas' "tolerance" of Colombian rebels, and Venezuela charging Colombia fails to take action against right-wing paramilitaries.