Colombian President Juan Manuel Santos has rejected calls by Farc rebels for a ceasefire once peace talks begin with the government in October.
A senior commander of the left-wing rebel group had said it was prepared to disarm and would seek a truce when the peace talks opened in Oslo, Norway.
The talks - the first direct negotiations in a decade - are then expected to move to Cuba.
Mr Santos said there would be no ceasefire and his government would not "lower its guard in terms of security".
He added that he did not want to "repeat the mistakes of the past" - an apparent reference to the failed peace talks in 2002, which reportedly gave time to Farc (The Revolutionary Armed Forces of Colombia) to regroup.
Government military operations against the rebels are continuing, with the authorities saying that a top Farc commander had been killed in an air strike.