Colombian President Juan Manuel Santos condemned the violence and asked the government's head negotiator in the ELN peace talks, Gustavo Bell, to return to Bogotá from Quito, Ecuador, reportedly to debate whether or not to continue the negotiations.
The United Nations said that its observers witnessed a major improvement for the civilian population in rebel territory since the truce came into effect in October.
The Colombian government has frozen dialogue with guerrillas of National Liberation Army (ELN) on Wednesday after this organization reattacked at end of a 101-day ceasefire yesterday.
"I deplore the decision by the ELN to reactivate its terrorist attacks", Santos said in a televised address.
Guterres' visit on Saturday and Sunday is to "support peace efforts" in Colombia, UN spokesman Stephane Dujarric said. The negotiator representing the rebels said the group had followed the ceasefire and that the two sides had agreed to evaluate a new deal after the first one was over.
The ministry said on Twitter that the nation's military has been ordered to resume operations against rebels "with full intensity and force".
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The three-month cease-fire between the army and the ELN ended on Tuesday.
U.N. Secretary-General Antonio Guterres is heading to Colombia this weekend to support peace efforts amid growing concerns about the reintegration of FARC rebels into civil society and new attacks by the last rebel holdouts.
During a Security Council meeting to discuss Colombia, Britain and Sweden urged Colombia and the ELN to resume ceasefire talks as quickly as possible.
However, the attacks "should not alter the course of the talks to achieve a political resolution to the conflict", the group said.
After the signing of the Peace Agreement in Colombia in November 2016, after some four years of talks in the Cuban capital, the United Nations Security Council approved a mission to verify the ceasefire and the delivery of weapons. "But peace comes with a will and concrete acts of peace, not just words", said Santos.
The government has also deployed 2,000 troops to Nariño's important port city of Tumaco to quell the fighting following the ceasefire.