A failure at a Venezuelan substation that supplies emergency electricity to Arauca, the capital of the same-named northeastern province on the border with Venezuela, left the city without electricity on Wednesday.
Arauca province has been the Colombian region most affected by rebel sabotage of the electric grid, regional officials said.
A spokesman for the Arauca Energy Company, or Enelar, told Efe by telephone that the electricity crisis in Arauca and other towns in the province got worse on Wednesday when the connection with Venezuela was cut and another electricity tower was toppled.
The tower is the second one knocked over since Tuesday night by the Revolutionary Armed Forces of Colombia, or FARC, guerrilla group, to which the army's 18th Brigade based in the provincial capital attributed the attacks in a communique.
Early on Wednesday, the rebels dynamited a second tower on the national interconnection line in Guamualito, a town near the border with Venezuela.
The attack disrupted electric service to the region, which had been affected on Tuesday night by an earlier attack in La Pesquera, where the FARC toppled another tower within the same interconnection system.
The first attack had left Arauca and five nearby towns without electricity, but Colombian state-run power grid operator Interconexion Electrica S.A. managed to get four of them - Saravena, Fortul, Tame and Puerto Rondon - back on line.
A binational agreement to supply emergency electricity from the Venezuelan town of Guasdualito allowed the regional capital to partially reestablish electric service.
However, the Enelar source said that a failure at the substation in the neighboring country on Wednesday had left the city without power again.
Some 100,000 residents of Arauca, Arauquita and Cravo Norte are being affected by the outage, which also temporarily affected the petroleum installations in the border region, which are dealing with the emergency with their own backup power plants.
Operating in the zone are state-run oil company Ecopetrol and U.S. oil firm Occidental Petroleum.
In a communique, ISA said that it had mobilized technicians to make inspections as a step toward restoring the affected lines.
For the past two weeks, the FARC has been conducting a campaign of attacks on Colombia's electrical infrastructure. The first attacks came in the southwestern part of the country but later strikes have been staged in the northeast and east.