Leftist rebels in Colombia began a unilateral ceasefire Saturday hailed as a key step in peace negotiations — but uncertainty marred the truce, which the guerrillas threatened to break if attacked by the army. “An indefinite, unilateral ceasefire from the FARC has begun. It’s a positive gesture that goes in the right direction,” said President Juan Manuel Santos, who consistently refused to reciprocate. Under the ceasefire, the Revolutionary Armed Forces of Colombia (FARC) says its fighters will only engage in hostilities if they come under attack first.