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BOGOTA (Reuters) - Colombia’s ELN rebel group must cease attacks and re-start peace talks with the government to end more than a half-century of war, United Nations Secretary General Antonio Guterres said on Sunday, amid renewed violence after the end of a ceasefire between the two sides.

The National Liberation Army (ELN) guerrilla group and the government of President Juan Manuel Santos have been in formal peace talks for nearly a year, but the rebels launched a fresh offensive last week, killing members of the security forces, bombing major oil pipelines and kidnapping an oil contractor after the expiration of a 101-day ceasefire.

In response to the renewed attacks, Santos recalled the head of the government’s team at the Quito talks to discuss the future of the negotiations.

“I urge the end of armed actions and the re-start of a serious and constructive dialogue with a view to fulfilling as soon as possible the commitment of the sides to reach a solution to the conflict by political means,” said Guterres, wrapping up his two-day visit to the Andean country to discuss the progress of its 2016 peace deal with the Revolutionary Armed Forces of Colombia (FARC) rebels as well as the talks with the ELN.

“There is no justification for the armed conflict in Colombia to continue,” he said, reiterating the U.N. Security Council’s call for the rebels and the government to fortify future ceasefires and avoid a return to fighting.

The ELN, founded by radical Roman Catholic priests in 1964, has sought peace with the government before but made little progress. It is considered a terrorist group by the United States and European Union.