United Nations Secretary-General Ban Ki-moon and Venezuelan President Nicolas Maduro arrived in Havana late Wednesday night ahead of the signing of an agreement between Colombia's government and leftist FARC rebels.
The two sides are slated to sign an agreement to end hostilities with a definitive ceasefire today.
After more than three years of fraught talks in the Cuban capital, the agreement brings into sight an end to a conflict that began as a 1960s peasant revolt before exploding into a cocaine-fuelled war that has killed at least 220,000 people and displaced millions.
The accord will be signed in Havana by President Juan Manuel Santos and Revolutionary Armed Forces of Colombia, or FARC, rebel leader Rodrigo Londono, better known by his nom de guerre Timochenko.
The agreement would leave the two sides just a step away from resolving the longest-running conflict in the Western Hemisphere.
"I congratulate (Colombian) President (Juan Manuel) Santos and the Colombian government. I congratulate the Revolutionary Armed Forces (of Colombia), the FARC and their commander, Timoleon for this historic advancement, this gigantic step. Peace in Colombia is our peace," Maduro said, referring to Londono by another of his aliases.
Ban and Maduro were expected to attend the signing ceremony along with Cuban President Raul Castro and Chilean President Michelle Bachelet.
The ceremony is expected to be held at 12:00 EDT (1600 GMT)