A unilateral ceasefire announced by the Revolutionary Armed Forces of Colombia (Farc) in November to allow peace talks with the government expired today.
President Juan Manuel Santos immediately warned the Farc not to resume operations.
He claimed that security forces had already detected "attempts to perpetrate acts of terrorism."
The Farc has said it is willing to extend the truce if the government enters into a mutual ceasefire, but Bogota has refused.
Mr Santos claimed last Monday that the group had only partially fulfilled its ceasefire.
Talks between government and Farc officials resumed in Havana this month, with both sides vowing to speed things up.
Farc delegation head Ivan Marquez has called on the government to "cease the warmongering rhetoric that accompanies false promises to resolve social problems."
But the government has continued to target the rebels, while accusing Farc of continued attacks.
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