BOGOTA, May 14 (Reuters) - Two Colombian undercover military officers who had infiltrated the country's largest rebel left-wing group have been tortured, killed and found dead over the border in Venezuela, the government said on Monday.

Colombia's Defense Minister Juan Manuel Santos said the killings were under investigation but the two men could have crossed into Venezuela as part of their undercover work against the Revolutionary Armed Forces of Colombia or FARC.

"They infiltrated the FARC; we have a lot of people infiltrated in the FARC. There is nothing unusual about that, and they were found dead in Venezuela," Santos told local Caracol radio.

"They were assassinated and appear also to have been tortured," he said.

Border incidents are a sensitive issue for the Andean neighbors. Some Colombian and U.S. officials have charged that left-wing President Hugo Chavez has allowed the FARC to use Venezuelan territory to shelter from army attacks.

Washington foe Chavez is a harsh critic of Colombia's U.S.-backed counter-narcotics and military aid program, which has been vital to Uribe's drive to reduce violence and the drug-trafficking that helps fuel the four-decade-old conflict.

The Venezuelan leader denies his government has aided the guerrillas and counters that Colombia has not done enough to prevent violent spillover from the conflict affecting its border areas. Kidnapping, smuggling and drug-trafficking are common along the porous 1,400-mile (2,200 km) frontier.

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