Fighters from Colombia’s main leftist rebel movement attacked an army patrol from Venezuelan territory on Monday, killing at least 12 soldiers and wounding 4 others, Colombian officials said. President Hugo Chávez of Venezuela responded quickly, saying he had sent additional troops to the region to ensure that no rebels passed into his country’s territory. “We are not going to permit irregular groups of whatever stripe to use Venezuela as a place to camp, train or attack forces of other countries, in this case Colombia,” Mr. Chávez told Venezuelan state television. The rebel group, the Revolutionary Armed Forces of Colombia, or FARC, has long used Venezuela as a sanctuary for its guerrillas. Ariel Ávila, an expert on the FARC at the research institute Nuevo Arco Iris, said there had been about 550 attacks by FARC rebels in the first four months of this year, an increase of about 3 percent from a year earlier and 15 percent from four years ago. He said the rebels were interested in regaining territory lost in the 1990s in La Guajira Province, where they extort money from ranchers and miners.