The two groups have deep ideological differences and have often clashed in the past but now appear willing to consider uniting for the sake of survival.

The FARC-ELN statement was released shortly after Colombia's air force said it had killed a key FARC commander and nine of his bodyguards in a bombing raid carried out in the mountainous northwest of the country.

The death of Ruben Garcia, a FARC leader also known as Danilo, was the latest blow to the guerrilla movement.

Rebels weakened

"You don't answer terrorists with anything other than force"

Francisco Santos, Colombian vice president

The rebel groups have been steadily weakened by battlefield defeats and record desertions since 2002, when Uribe first came to power promising to step up the offensive against rebel fighters.

The ELN, which claims to have 5,000 fighters, was formed by left-wing Roman Catholic priests inspired by the liberation theology movement in the 1960s.

The FARC, according to official estimates, has about 9,000 fighters and started as a hardcore pro-Soviet communist organisation before getting involved in extortion of local communities, drug trafficking and kidnapping.

Francisco Santos, the Colombian vice-president, was dismissive of the group's threat to escalate their fight against the government.

"You don't answer terrorists with anything other than force," Santos told reporters in response to the FARC-ELN statement.

Many analysts say the two groups have become increasingly isolated because of the government crackdown and that it is too late for the groups to mount any significant threat, adding that the rebels are only considering an alliance as a matter of survival.