BOGOTA (Reuters) - Colombia on Wednesday warned Venezuela not to brush aside last month's deaths of nine Colombians on the border as paramilitary gang violence, an incident that fueled tensions between the Andean neighbors.

Ties have frayed over last month's border killings, the murder of two Venezuelan soldiers near the frontier and accusations from President Hugo Chavez that three Colombian agents were caught spying on his anti-U.S. government.

Venezuela and Colombia often spar over Colombia's conflict spilling over the frontier. But the current feud is damaging $7 billion a year in trade after Chavez suspended ties and reinforced border security.

Venezuela blamed last month's kidnap and murder of the group of Colombians on paramilitaries and says illegal Colombian militias also killed the two of its soldiers gunned down on Monday by assassins riding motorcycles.

"Any hypothesis about the deaths of the Colombians in Venezuela is very serious," Foreign Minister Jaime Bermudez said in a statement. "Some in that country are trying to suggest if these men were members of so-called paramilitaries then there is some justification for what happened."

Violence is common along the porous, 1,375-mile (2,200-km) border, where outlawed Colombian militias, leftist guerrillas, drug traffickers and contraband smugglers operate.

Tensions have increased over Colombia's deal with the United States to allow U.S. troops more access to its military bases as part of cooperation against drug traffickers and leftist rebels fighting Latin America's oldest insurgency.

Colombian President Alvaro Uribe, a staunch Washington ally, says the accord extends existing military cooperation. But Chavez has warned the military bases could be used to launch an U.S. offensive his OPEC nation.

Venezuelan authorities are now holding three men they accuse of spying for Colombia's DAS state security agency. But Colombia says only one of the men is a DAS agent, who was invited over the border by a Venezuelan colleague.