Colombia's leftist rebel group Farc has suspended a unilateral ceasefire after 26 of its fighters were killed in a government air andground offensive.
The ceasefire had been in place since December 2014.
President Juan Manuel Santos called on the rebels to "speed up" negotiations taking place in Havana, Cuba.
The two sides have been in on-off negotiations since 2012 in an attempt to end more than half a century of conflict.
Thursday's attack - in the country's south-western Cauca region - was the deadliest since the resumption of air strikes against the rebels last month.
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.
'How many more deaths?'Mr Santos, who restarted air strikes against the rebels last month after 11 soldiers were killed in an ambush, addressed the rebels via his Twitter account.
He wrote: "Gentlemen of the Farc: now is the time to speed up the negotiations. How many more deaths do we need to understand that we have arrived at the time for peace!" he wrote.
Thursday's military action comprised an air strike and ground assault against the rebel group in Guapi, Cauca, said officials.
The operation's aim was the capture of a rebel known by the alias "Javier", and two rebels were captured along with those killed, defence ministry officials told the Efe news agency.
At the previous round of peace talks, Farc called for the dismantling of right-wing paramilitary groups fighting against the rebels.
The two sides failed to reach an agreement over reparations for the victims of the conflict, an issue that has been wrangled over for months.