The fighting has killed more than 40,000 people in the last decade

Some 2,000 soldiers, supported by helicopter gunships, were sent to Bolivar province - an ELN stronghold, the army said.

This week's violence was sparked by the Colombian Government's suspension of peace talks in Venezuela with the ELN guerrillas.

At least three people were killed and up to 42 people injured in a blast in the north-eastern town of San Francisco, and on Thursday the rebel force detonated bombs in two other towns in northern Colombia.

'Armed strike'

In a separate incident on Friday, six ELN rebels died in clashes with the army in the northern province of Magdalena, the army said.

In a BBC interview on Friday, the ELN commander, Gabino, said dialogue had to take place in Colombia - peace talks on foreign soil were unacceptable, he said.

The group also announced in a communique that its was calling an 'armed strike' of three days in Arauca and it was "not responsible for people or vehicles travelling in the department".

Talks between the ELN and the Colombian Government had been taking place in neighbouring Venezuela.

They were called off by Bogota earlier this week, after the rebels had raised new demands and rejected government proposals.