Two suspected FARC fighters were killed in combat in northern Colombia, four days after the government announced a revised peace accord with the rebel group.
"Fighting took place in the south of the Bolivar department, and guerrillas who claimed to belong to the FARC were killed," the government's chief peace negotiator told a television station.
"The Defence Ministry holds that there was an illegal operation there," he added.
The army confirmed the report of fighting which comes in violation of a bilateral ceasefire in place since August.
The army said in a statement that another suspected rebel who was detained said that he and the two killed were with the FARC's 37th Front.
The military also said, however, that locals insisted the insurgents were with the smaller National Liberation Army (ELN), which is often involved in kidnapping and extortion.
The government and the FARC had announced over the weekend a new peace deal to end more than half-a-century of violent civil strife.
Colombian voters had unexpectedly rejected an earlier version of the government's peace deal with the FARC in an October 2 referendum, deeming it too soft on the country's largest rebel group.
The government and the ELN, the second largest guerrilla group, had planned to launch public peace talks on 27 October in Quito.
President Juan Manuel Santos cancelled the negotiations after the rebels failed to release hostage ex-congressman Otis Sanchez.
The FARC and the ELN are the last two leftist guerrilla groups involved in a multi-sided conflict in Colombia that has killed more than 260,000, left seven million displaced and 45,000 missing.