Colombia's government and its largest rebel group signed a new, modified peace accord in Havana on Saturday following the surprise rejection of an earlier deal by voters in a referendum.

The latest agreement aims to address some of the concerns of opponents of the original accord, who said the deal was too lenient on a rebel group that had kidnapped and committed war crimes.

"The new deal is an opportunity to clear up doubts, but above all to unite us," said government negotiator Humberto de la Calle, who signed the accord along with rebel negotiator Ivan Marquez, moving to end a half-century-long conflict that has claimed more than 220,000 lives and driven almost 8 million people from their homes.

De la Calle described the text of the modified accord as "much better" than the previous one, but didn't say if or how it would be submitted again to voters or to congress.

President Juan Manuel Santos and the Revolutionary Armed Forces of Colombia inked an initial peace deal on Sept. 26 amid international fanfare after more than four years of negotiations. But voters rejected it on Oct. 2 by just 55,000 votes, dealing a stunning setback to Santos who won the Nobel Peace Prize for his efforts to end Colombia's conflict.

Santos immediately began looking for ways to rescue the deal and the sides extended a cease-fire until Dec. 31 to get the modified deal done. The rebels insisted they wouldn't go back to the drawing board and throw out years of arduous negotiations with the government.

"The meetings with the FARC delegation were intense," said De la Calle. "We worked 15 days and nights to reach this new agreement."

De la Calle said some modifications made were related to justice, punishment for combatants accused of war crimes and reparations for the conflict's victims. He said negotiators had worked out the details of how and where those responsible for crimes would serve their sentences, addressing complaints by opponents that rebels accused of atrocities would not be imprisoned but submitted to "alternative punishments."

Other modifications include requiring the rebels to present an inventory of acquired money and holdings, and the provision of safeguards for private owners and property during reforms carried out in the countryside. Cases of conflict participants accused of drug trafficking would be dealt with under Colombia's penal code and be heard by high courts.

FARC negotiator Marquez said "the implementation of the accord is all that remains for the construction of the bases for peace in Colombia."

Hours before the deal was announced, Uribe, who was Colombia's president from 2002 to 2010, had asked that it "not be definitive" until opponents and victims of the conflict could review the text. Uribe and his supporters had demanded stiffer penalties for rebels who committed war crimes and criticized the promise of a political role for the FARC, a 7,000-strong peasant army that is Latin America's last remaining major insurgency. They didn't like that under the old deal guerrilla leaders involved in crimes against humanity would be spared jail time and allowed to enter political life.