The head of Colombia's last remaining rebel group has ordered fighters to stand down in advance of a bilateral ceasefire taking effect this weekend.

The ceasefire is an important milestone in efforts to end a half-century of political violence in the South American country.

In a rare video posted online on Friday, Nicolas Rodriguez, known by his war alias Gabino, said he was confident fighters belonging to the National Liberation Army (ELN) would obey orders not to attack government troops or take other actions banned by the ceasefire.

"I have no doubt of your loyalty and that you will fulfil this commitment to the last consequences," Rodriguez said.

Hopes were high for peace talks between the government and the ELN that started in February in Ecuador following an agreement last year that led to the disarmament of the much larger Revolutionary Armed Forces of Colombia (FARC).

But progress has been slow as the ELN has remained on the offensive and stepped up attacks on oil pipelines.

Just this week, the group was blamed for two simultaneous attacks on a major oil pipeline near the border with Venezuela that dumped crude in several rivers and farms affecting 700 families.

President Juan Manuel Santos on Friday signed a decree ordering troops not to attack ELN rebels once the temporary ceasefire takes effect at midnight Saturday.

Santos pointed out that this would be the first bilateral ceasefire the ELN has signed in its history.

The ceasefire, approved on September 4, will initially last until January 9, after which it can be renewed by mutual consent.

As part of the agreement, the ELN has pledged to suspend hostage-taking, recruitment of minors and attacks on infrastructure.

INFOGRAPHIC: Colombia - FARC and ELN areas of influence

The government, in turn, must improve conditions for jailed fighters, as well as boost protections for leftist activists in areas dominated by the ELN.

"As Pope Francis has urged, we must never stop pursuing peace," Santos said.

According to the protocol signed by government and rebel negotiators, UN observers will fan out across the country to 33 areas where the ELN is strongest to monitor the ceasefire's implementation.

They will be assisted by the Roman Catholic Church, from which many of the ELN's founders - priests and seminary students - hail.

The FARC and ELN formed in 1964 to fight for land rights and protection of poor rural communities.