Leftist National Liberation Army (ELN) rebels in Colombia have killed 12 security officials in Boyaca state bordering Venezuela, President Juan Manuel Santos has said.

"While we had one of the least violent election days in years (Sunday), the ELN today has slaughtered 11 army troops and a police officer in Boyaca," Santos said in a televised address from the presidential palace on Monday.

The attack occured as troops were transporting to the capital, Bogota, for counting about 130 ballots cast at the remote Uwa reservation during Sunday's elections for governors, mayors and other local officials.

Three more soldiers were wounded and six people remain missing, including two poll workers and an indigenous guide, Defence Minister Luis Carlos Villegas said.


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"They were safeguarding the political liberty of our Uwa brothers," Villegas said in a press conference, adding that the military's top command had traveled to the area to oversee efforts to locate those still missing as well as the attackers.

The president warned he had ordered the defence ministry and the military to "step up" anti-ELN operations.

"This shows that the ELN does not understand that this is peace time, and not time for war," Santos said.

"If the ELN thinks that with these acts it is going to gain political space, or leverage in a possible negotiating process, it is dead wrong," he added.

The Colombian government, which has inked peace deal with the larger FARC guerrilla force, has been trying to bring the ELN to table.

The Santos government has been engaged in peace negotiations with the FARC since 2012, which the sides aim to wrap up with a final peace deal by March 2016.

After announcing that timetable last month, Santos encouraged the ELN, which has some 2,500 members compared to the FARC's 7,000, to follow the FARC's lead in seeking a peace deal.

The ELN has participated in preliminary peace talks with the government since 2014, but has not yet opened a formal peace process.

Source: AP And AFP