Ten Colombian soldiers have been killed in an attack by left-wing Farc rebels in west of the country, officials say.
The government says 17 soldiers were also injured, four of them seriously, following an attack in the Farc stronghold of Cauca.
President Juan Manuel Santos says he will travel to the area on Wednesday.
Analysts describe it as the biggest violation of the current ceasefire between the two sides as they hold peace talks in Havana, Cuba.
On Twitter, President Santos wrote, “We deeply regret the dead of our soldiers. That’s precisely why we want to end the war”
The soldiers were attacked at dawn at an army garrison in Buenos Aires, Cauca, according to local governor Temistocles Ortega.
The army says the rebels used explosives, grenades and firearms.
Decades of war
Farc, the largest of Colombia’s rebel groups, was founded in 1964 with the aim of installing a Marxist regime.
Its negotiators have been engaged in peace talks with the Colombian government since November 2012. Farc declared a unilateral ceasefire in December 2014 as a show of faith in the talks.
This led to the government halting bombing raids against the rebel camps in March, a truce that President Santos extended only last week.
It is unclear how this attack will affect the Havana peace talks with some arguing it is proof that Farc cannot be trusted.
BBC Mundo’s Arturo Wallace reports that critics are also calling for use of military force against Farc in response.
There are other guerrilla groups in the country that remain in conflict with the government.
More than 220,000 people are estimated to have died in five decades of conflict.
Farc is listed as a terrorist organisation by the US and Europe.
Timeline of the peace process
November 2012 – Formal peace talks begin in the Cuban capital Havana between the Colombian government and the Farc
May 2013 – A deal is reached on land reform, one of the most contentious issues. It calls for fair access to land, and rural development, two key causes of the conflict
November 2013 – The two sides agree on the political participation of the Farc should a peace deal be reached
May 2014 – Both parties pledge to eliminate all illicit drug production in Colombia
December 2014 – The Farc declares a unilateral ceasefire
March 2015 – Both sides agree to work together to remove landmines. Colombia is one of the most heavily mined countries in the world