Colombia's government and the Revolutionary Armed Forces of Colombia have started talks on how to rework a peace deal, rejected in a referendum last month, that would end Latin America's longest civil war.
"Meeting of delegates and advisers to government and FARC in Havana. Starting constructive dialogue. Let's get peace done," the government said on Twitter.
"It's an optimistic atmosphere. Let's get peace," the lead FARC negotiator Timoleon Jimenez, known as Timochenko, tweeted.
The sides are "trying to find common ground," he said.
Mr Santos, who has staked his legacy on making peace, had extended the army's ceasefire to 31 December if no solution to the impasse is found by then.
FARC, which had criticised Mr Santos's deadline, has also confirmed its willingness to continue negotiations and maintain a bilateral ceasefire.
The Colombian leader won the Nobel Peace Prize just a few days after voters shot down the historic accord in a referendum that would have ended more than 52 years of conflict.
Since the accord's rejection on 2 October, Mr Santos has held marathon talks with political figures including the country's former president Alvaro Uribe, who led opposition to the agreement, as well as religious leaders and victims of the armed conflict.
Under the peace accord that was rejected in the referendum, FARC's estimated 7,500 fighters are to disarm under UN supervision.
Mr Santos launched talks with FARC after taking office in 2010, with the two sides sealing a historic deal on 24 August to end the conflict, which has claimed 260,000 lives.
But there is no ceasefire in place with the much smaller National Liberation Army.
The FARC and ELN have been at war with the state since 1964.
The ELN is estimated to be about a quarter the size of the FARC, with 1,500 fighters.