On Saturday, Colombian President Juan Manuel Santos said that the security forces were ready to respond to rebel attacks, urging FARC members not to mount "terrorist attacks."
According to the Colombian military, rebels have violated the truce at least 52 times over the past two months.
The Colombian think tank New Rainbow Corporation also reported that at least nine battles have been initiated by the rebels since the beginning of the truce.
The conflict began in 1964 when FARC was established as a communist agrarian faction to protest against the concentration of land ownership and social inequalities in Colombia.
The Revolutionary Armed Forces of Colombia (FARC) says it is ending a two-month-long unilateral ceasefire with the Colombian government after the government said it will not extend the truce with the rebel force.
"With heartache again we must admit the return of the military phase of the war, which nobody wants," AFP quoted FARC chief negotiator, Ivan Marquez as saying on Sunday before going into a new session of talks with Colombian government representatives in Havana.
Marquez, the FARC number two, pointed out that the group had been willing to extend the truce, due to expire at midnight on Sunday (0500 GMT Monday), if the Colombian government had agreed to a bilateral truce.
"I want to take this opportunity once more to ask the government of Colombia to study the possibility of analyzing a bilateral ceasefire and [an end to] hostilities so that we can surround these peace talks with an environment of peace," he said.
Colombia’s largest rebel force, FARC, announced a two-month-long ceasefire with the Colombian government in November 2012 at the outset of talks in Cuba aimed at ending Latin America's longest-running insurgency.
The Colombian government, however, has continued to target rebels, arguing that the leftist group would use ceasefire as an opportunity to rearm.