Santos said on Twitter that the government was "always willing to extend the ceasefire", but the rebels had "inexplicably refused and today terrorist attacks resumed".
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.
U.N. Secretary-General Antonio Guterres is heading to Colombia this weekend to support peace efforts amid growing concerns about the integration of the country's largest rebel group and new attacks by the last rebel holdouts.
The talks are complicated by a lack of confidence between the parties and upcoming votes to elect a new president and congress. "My commitment to peace has been and will be unwavering".
At the council meeting, the United Nations envoy for Colombia, Jean Arnault, said the clamor for a suspension of military action by the ELN has been unanimous throughout the country, "notwithstanding the many imperfections of the cease-fire".
However, the attacks "should not alter the course of the talks to achieve a political resolution to the conflict", the group said. They are scheduled to discuss the current state of peace efforts in the Latin American country as well as Colombia's efforts to reintegrate FARC rebels into society. "I think that our government has demonstrated to the entire world that we, in particular our president, are proponents and champions of peace". FARC rebels reached an historic peace agreement ending the region's longest-running conflict in 2016, an event that many credit with paving the way toward ELN peace talks.
The ELN, however, also called for the peace talks to continue and insisted that the violence had occurred in a "complex situation".
Under the temporary agreement, the 1,500-member ELN pledged to renounce hostage-taking, recruitment of minors and attacks on infrastructure. In a separate attack near the frontier, a sniper killed a Colombian soldier.
The government has also deployed 2,000 troops to Nariño's important port city of Tumaco to quell the fighting following the ceasefire. The government in turn vowed to improve conditions for jailed rebels as well as boost protections for leftist activists in areas dominated by the ELN.
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