The guerrilla group said in a statement that the ceasefire suspension was the result of "five months of land and air offensives against our structures all over the country".
"We deplore the joint attack by the air force, the army and the police," said the statement.
"We feel pain for the deaths of guerrilla fighters as well as soldiers, sons of a same people and from poor families".
But the group said they were willing to continue with the latest round of peace talks in Havana.
"Against our will we will proceed with the [peace] talks in the middle of a confrontation," they said.
They also reiterated their demand for Mr Santos to agree a bilateral ceasefire.