Fighting broke out over the weekend in Arauca - which sits on the border with Venezuela - as members of the National Liberation Army (ELN) fought with dissidents of the Revolutionary Armed Forces of Colombia (FARC), who reject a 2016 peace deal.
They were fighting over control of illegal economies such as drug trafficking, Colombia's army said in a statement.
The violence harks back to the mid-2000s, when the FARC and ELN fought each other in Arauca and the neighbouring Venezuelan state of Apure.
By the time fighting ceased in 2010, more than 58,000 people had been displaced in the province and at least 868 civilians had been killed, according to a report from advocacy group Human Rights Watch, which cited the government-run Colombian Victims' Unit.
Some 5000 people fled Apure at the end of March last year amid clashes between Colombian armed groups and the Venezuelan military.
So far, HRW has received reports of 24 deaths due to the violence, the group's senior investigator for the Americas Juan Pappier said in a message on Twitter, adding there had also been forced displacements and kidnappings.
"We are very concerned about the fighting between the ELN and dissidents of the FARC's 10th front in Arauca and Apure," Pappier said.
Colombia's President Ivan Duque convened a meeting of military and police leaders to assess the situation.
"I have ordered that two battalions be deployed within the next 72 hours to help with the task of territorial control," Duque said in a video broadcast.
Colombia's government accuses Venezuela's President Nicolas Maduro of harbouring FARC dissidents and ELN combatants, something Maduro has repeatedly denied.
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