Colombia and leftist FARC guerrillas concluded a key stage in talks on Friday aimed at ending five decades of war, as the rebels announced a truce for the upcoming presidential election.

The outcome of talks between the Revolutionary Armed Forces of Colombia (FARC) rebels and Bogota authorities is important for President Juan Manuel Santos's May 25 re-election bid.

Santos faces growing opposition to the peace talks from conservatives, especially Oscar Ivan Zuluaga, a top candidate supported by popular former president Alvaro Uribe.

FARC members said the May 20-28 unilateral truce will include Colombia's second-largest guerrilla group, the National Liberation Army (ELN). Government officials however said they will continue to pursue the rebels.

Colombian and FARC representatives have been negotiating a peace deal in communist Cuba since November 2012.

On Friday the two sides agreed on ways to end Colombia's vast illicit drugs trade, the third of a six-point peace agenda.

-- 'Closer' to peace --

"Today we are very close, closer than ever to obtaining peace," Santos told supporters in Bogota.

"This is a definite step, an important step, and great news for Colombia and for the whole world."

Rodolfo Benitez of Cuba, whose country is a guarantor of the process, said the deal would see a campaign to eradicate illicit crops such as coca -- the source plant of cocaine -- and heroin. The plants will be forcefully eradicated if farmers insist on growing them.

Colombia and Peru are the world's top producers of cocaine, a product that comes exclusively from coca bushes grown on the eastern slopes of the Andes.

The drug trade has fueled the leftist insurgency as well as right-wing paramilitary groups and organized crime.

Norwegian diplomat Dag Nylander, whose country is also a guarantor, said that Colombia would convene an international conference against drug trafficking under the aegis of the United Nations.

"Both the government and the FARC acknowledge that the drug trade has fed the conflict in Colombia," lead government representative Humberto de la Calle said.

Negotiators earlier agreed on issues of rural development and guerrilla integration into the political process.

Still to be discussed: how FARC fighters will lay down their weapons, compensation for conflict victims, and determining whether a final peace deal should be put to a national vote, as the government wants.

Colombia specialist Christian Voelkel with the International Crisis Group told AFP that the agreement was a landmark moment in the peace process, and that both sides were now past the point of no return.

- 'Not going to stop' -

The FARC, which has between 7,000 and 8,000 fighters, have been at war with the state since its founding in 1964. Hundreds of thousands of people have died in the conflict.

They said their unilateral truce for the election period was a gesture of goodwill.

But Defense Minister Juan Carlos Pinzon vowed there would be no let-up -- Bogota has repeatedly refused to agree to a ceasefire during the peace talks because the rebels have used previous ceasefires to rearm and regroup.

"We are not going to stop pursuing them simply because they do us the favor of refraining from committing one of the many crimes they commit," Pinzon said.

FARC representative Pablo Catatumbo said the rebels reject the legitimacy of Colombia's electoral system. "Nevertheless, we believe that a national clamor" for peace "should be heeded," he said.

The FARC has held temporary unilateral ceasefires twice before since peace talks started. However this would be the first time the smaller ELN - which also wants to start peace talks with the government - joins in a truce.