Civilians were "likely" among the casualties from clashes between members of Colombia's last recognized guerrilla group, the ELN, and dissidents of the FARC rebel movement, President Ivan Duque said.

"We have found 23 dead" in a rural area of the northeastern Arauca department, deputy defense minister Jairo Garcia said, without mentioning civilian victims.

Duque blamed his counterpart Nicolas Maduro for harboring Colombian rebels on the Venezuelan side of the countries' 2,200-kilometer border.

National Liberation Army (ELN) fighters and dissidents of the Revolutionary Armed Forces of Colombia (FARC) who rejected a 2016 peace pact "have been operating at will in Venezuelan territory with the consent and protection of the dictatorial regime," the president said.

Colombia's ombudsman Carlos Camargo said a dozen families have had to flee fighting in the municipalities of Tame and Saravena in the Arauca department.

Local authorities have reported that some communities are trapped in the crossfire.

The Americas director of Human Rights Watch, Jose Miguel Vivanco, said on Twitter Monday that the situation on the border was "very serious."

Colombia frequently accuses its neighbor of harboring rebel fighters.

The countries broke off diplomatic ties shortly after Duque took over as president from Juan Manuel Santos in August 2018, two years after a peace agreement ended a near-six decade internal conflict.

Violence has soared in Colombia despite the deal which saw some 13,000 FARC guerrillas lay down arms and the movement transform itself into a minority political party.

But dissidents are continuing to fight paramilitary groups, drug gangs and rebels for control of trafficking and illegal mining markets in the world's largest cocaine-producing country.

There are thought to be about 2,500 active ELN fighters and more than 5,000 FARC dissidents in operation -- the majority of them new recruits.