The status of belligerency is recognised under international law and would grant the two groups semi-legitimacy and more rights.
The move will anger Colombia, which earlier this week rejected Chavez's calls to remove the terror label.
Chavez has yet to formally announce the new status.
Relations between the two nations are already strained over moves by Chavez to release scores of hostages held by Farc. 'Sabotage' claim
Chavez, who has yet officially confirm said on Wednesday that the Revolutionary Armed Forces of Colombia (Farc) and the ELN, were "not any terrorist body but ... real armies that occupy territory in Colombia".
"They must be recognised, they are insurgent forces that have a political project ... which here is respected."
On January 10 Chavez helped engineer the release of two Colombian politicians held by Farc for over five years.
However he says he does not support either Farc or the ELN but merely wants to help bring about a peace deal between all of the sides in Colombia's civil conflict.
|Diplomatic relations between Colombia and |
Venezuela have been strained recently [AFP]
The Colombian government has accused him of meddling however.
Fernando Araujo, Colombia's foreign minister, said that Chavez "doesn't miss a chance to mistreat Colombia, its government and Colombian leaders, while he is unaware of the guerrillas' acts of terrorism, their involvement in drug trafficking and crimes against humanity".
The Venezuelan foreign ministry responded on Thursday with a statement accuusing Colombia of "sabotaging the humanitarian missions led by the international community, putting the lives of innocent people at risk."
The tensions come as a group of relatives of captives held by Farc arrived in Venezuela on Friday to ask Chavez to mediate their release. US concern
Adam Isacson, from the Centre for International Policy, told Al Jazeera that Chavez's move was mainly symbolic, although it could make negotiations with the group easier for other nations.
The US has expressed concern at Chavez's comments, with Navy Admiral Mike Mullen, the US chairman of the joint chiefs of staff, saying on Thursday during a visit to Colombia that Chavez's strategic backing of the Farc is of concern.
However, he added that he was not aware of any specific logistical support by Chavez for the rebels.
Colombia accuses the Farc of holding more than 700 hostages, many of whom have been held for years in the jungle.
The Farc are Marxist revolutionaries fighting to overthrow the government through armed struggle.
Their favoured methods of kidnapping and bomb attacks have led critics to accuse them of human rights violations.
For years Farc, the ELN, paramilitary groups allied to the country's drug trade have fought Colombian government forces leaving thousands of people dead.