But though some hostilities have resumed, both sides say they will continue to take part in negotiations to end the conflict.
The peace talks are being held in Cuba, with the goal of bringing an end to the leftist FARC's war against the government, which began in the 1960s, making it the longest-running insurgency in Latin America.
"It was not in our plans to suspend the unilateral and indefinite ceasefire proclaimed on December 20, 2014 as a humanitarian gesture of de-escalation of the conflict," the FARC said in a statement.
The group blamed the "inconsistency" of President Juan Manuel Santos' government and its attacks on FARC outposts for its decision to end the ceasefire.
In the course of the talks, both sides have implemented and retracted unilateral ceasefires several times. The spark behind the latest renewal of conflict may threaten to derail or slow the talks.
In December, the FARC announced a unilateral and indefinite ceasefire. In March, President Santos announced a month-long temporary ceasefire in an attempt to de-escalate the conflict.
Then in April, a FARC attack on a group of Colombian soldiers in the southwestern province of Cauca killed 11 and wounded 19.
The government accused the FARC of violating its ceasefire and resumed its operations against the rebels. Throughout the peace talks, the government's default position has been to continue operations against the FARC even while it negotiates with them.
A Colombian military raid killed 26 FARC militants in Cauca on Thursday, in retaliation for the April ambush.
According to local media, the FARC claimed that the April attack against the government troops was not a pre-meditated action by the rebel leadership.
But after the government's attack on the FARC's Cauca position, the rebels said they have no option but to take up arms.
"For us, the deaths of guerrilla combatants and soldiers are equally painful; they are children of the same nation and from poor families," FARC said in a statement. "Although Santos announces that he will keep the offensive, we insist on the need to agree, as soon as possible, to the health of the peace process and to prevent further victimization, on a bilateral ceasefire, insistently requested by national majorities."
Outside observers have also suggested that a bilateral ceasefire is needed.
President Santos, via Twitter, responded: "Men of the FARC: It is time to speed up the negotiations. How many more dead do we need to understand that the time for peace has arrived!"
Santos announced the formal peace talks in September 2012. The effort has been lauded by other nations, though it remains controversial in Colombia. Past efforts at peace have ended in disaster.
But despite the start-and-stop fighting of the last several years, the peace talks in Havana have persisted. Of the six major points identified by the participants, agreements have been reached on three: land reform, political participation by rebels, and the elimination of illegal crops.
The points that remain to be discussed are the rights of the conflict's victims, the disarming of the FARC, and the plan for how a final agreement will be enforced.