Colombia's Farc rebels have reached a landmark agreement with the government toward eliminating the illegal drug trade and called a week-long ceasefire, giving a political boost to President Juan Manuel Santos in his re-election bid.

The unilateral ceasefire includes the first round of presidential elections on May 25. It was announced after Santos, once a clear favourite, began to falter in public opinion polls with the rise of right-wing rival Oscar Ivan Zuluaga. The voting is expected to go to a second-round runoff on June 15.