The Colombian army says it has killed a rebel from the National Liberation Army, or ELN, and arrested four others in the northern province of Casanare.
The attack was launched 10 days before the beginning of peace talks between the government and the Marxist rebels.
There is no indication that either side will declare a ceasefire until considerable progress has been made.
During talks with another rebel group, the Farc, the government refused to stop fighting until a deal was done.
Twenty-four ELN rebels agreed to demobilise in Casanare and rejoin civil society, Colombia's Defence Ministry said in a statement.
The operation targeted the rebel group's public order and financial wing, the authorities informed.
The ELN and the Colombian government have agreed to open formal negotiations in neighbouring Ecuador on 27 October.
They have been engaged in an armed conflict for more than five decades.
The ELN says it will not be rushed into a peace agreement and that it wants civil society to be involved in the search for a permanent peace deal.
The Marxist rebel group said it rejected an "express peace process", after the Colombian government pushed for speedy peace talks.
The Colombian government for its part is keen to reach a deal with the ELN as soon as possible.
Who are the ELN rebels?
- The guerrilla group was founded in 1964 to fight Colombia's unequal distribution of land and riches, inspired by the Cuban revolution of 1959.
- Over the decades, the group has attacked large landholders and multinational companies, and repeatedly blown up oil pipelines.
- To finance itself it has resorted to extortion, kidnappings and drug trafficking.
- It has been strongest in rural areas.
The government is still reeling from the rejection by Colombians of a peace agreement with the country's largest rebel group, the Revolutionary Armed Forces of Colombia (Farc).
That agreement was reached after almost four years of talks in the Cuban capital, Havana.
But it was rejected by a narrow margin in a popular vote on 2 October by Colombians, many of whom thought it was too lenient on the Farc.
Government and ELN rebel negotiators announced back in March that they would open formal peace negotiations.
But the talks, originally scheduled to start in May, were delayed after the ELN rebels failed to meet the government's demand that it stop kidnapping people.
The ELN has since made a commitment not to carry out any more kidnappings, Colombian President Juan Manuel Santos said.
The group has released three hostages over the past weeks and is expected to free another two before the start of the talks in Quito.