US Secretary of State John Kerry has met separately with the Colombian government and the nominally Marxist FARC guerrilla group. He called for the factions to "redouble" their efforts to resolve their remaining issues.
The secretary of state, in Cuba as part of US President Barack Obama's historic visit, met with the two sides of the 51-year conflict on the sidelines of the trip.
Kerry told both the Colombian government and representatives of the Revolutionary Armed Forces of Colombia (FARC) that he was encouraged by progress in the peace process, according to spokesman Mark Toner.
In December, the two sides agreed a deal regarding reparations for victims. A final peace agreement to end the region's longest running war had been expected Wednesday, but has been postponed.
Kerry added that he was "encouraged that the 'end of conflict' issues are now front and center in the negotiations, including a formal bilateral ceasefire monitored by the UN Security Council, a timetable for disarmament and security guarantees post-conflict for all lawful political actors," the statement said.
Kerry's involvement came at the request of Colombian President Juan Manuel Santos and marked the first time a US secretary of state had met with FARC negotiators since peace talks started in the Cuban capital over three years ago.
Afterward, FARC negotiating chief Ivan Marquez tweeted about the "historic meeting," adding that Colombia's peace process was moving forward.
Marquez said he agreed with Kerry that neither side could afford to let the opportunity for peace pass.
Final deal remains elusive
Earlier this month, FARC leaders said peace talks were stalled over the issue of a bilateral ceasefire.
A final deal to turn FARC into a political party and end the ferocious conflict remains elusive despite several key advances in recent months.
Hostilities have almost entirely halted under the FARC's unilateral ceasefire, although a smaller rebel group, the National Liberation Army, has not joined the peace process and continues periodic attacks.
Since 1964, Colombia's war has uprooted 6.6 million people and killed more than 260,000.
The US sees the peace talks hosted by Cuba as an example of how restoring normal relations with Havana can help it achieve its wider goals in Latin America.