The Colombian government and the nation’s main rebel group said on Saturday that they had reached a “new final accord” to end their longstanding conflict, potentially reviving a deal that was rejected last month in a referendum.

The changes to the agreement with the rebel group, the Revolutionary Armed Forces of Colombia, or the FARC, were announced in Havana and addressed a range of topics — such as where rebels would be confined after disarming and how courts might address drug trafficking offenses — that negotiators said had troubled voters.

The agreement also appeared to withdraw a promise of guaranteed seats for rebels in Congress — one key demand of those who said the rebels would be unfairly rewarded with political positions.

In a televised address, President Juan Manuel Santos emphasized the need to ratify the agreement quickly, saying time was running out. “The cease-fire is fragile,” he said. “The uncertainty generates fears and increases the risks to throw this immense effort overboard.”