The Colombian government and the country's biggest rebel group announced an agreement Sunday on one of their main bones of contention, land reform, after more than a half year of slow-moving peace talks in Cuba's capital.

Both said the agreement constituted a major breakthrough, although several key details still needed to be worked out in the coming weeks and months. They did not release the text of the accord, but said it dealt with issues like property rights, access to land and rural infrastructure development.

"This agreement will be the start of a radical transformation of the countryside," the joint communique said.

The parties must now reach understandings in five other areas, starting with the political reintegration of fighters for the rebel movement, the Revolutionary Armed Forces of Colombia, another highly sensitive issue.

The two sides have stressed that no concessions are final until a complete peace accord is reached. But for one day, at least, the long-time enemies seemed optimistic an important step had been taken toward ending the half-century long conflict.

"Today we have a real opportunity to attain peace through dialogue," said the government's chief negotiator, Humberto de la Calle. "To support this process is to believe in Colombia."

Colombian President Juan Manuel Santos added in a tweet from Bogota: "We celebrate, truly, this fundamental step taken in Havana toward a full agreement to put an end to half a century of conflict."

Both sides say land rights lie at the heart of Colombia's armed conflict. About 2 million hectares (5 million acres) of land has been stolen from rural farmers by armed groups during the conflict, with twice that amount abandoned by those fleeing the violence.

The rebels say some land has also been expropriated by officials in fraudulent and corrupt processes, while the government says the FARC has also forced people off land.

Analysts and aid groups say much of the stolen land was taken by far-right paramilitary groups working in the service of ranchers and drug traffickers, while the government says complaints it received indicate at least a third was taken by the FARC -- a claim the rebel group denied. A lot of land was laundered through straw buyers and purchased by agribusiness operations that grow high value crops for export.

More than five dozen people have been slain trying to regain lost land over the last five years, according to the non-governmental group Forging the Future Foundation. The government and aid workers say five land claimants have lost their lives so far this year.

Despite Sunday's deal, rebel chief negotiator Ivan Marquez said several issues surrounding land reform are unresolved.

A senior official involved in the talks, who agreed to discuss the matter only if not quoted by name, said the final points of disagreement revolved around the exact amount of territory that will be involved, though both sides say it will affect millions of hectares (acres). Another dispute centers on a rebel demand to limit the amount of land held by foreigners.

Those questions and several others will be re-examined as both sides take up other issues, none expected to be as vexing. The agreement on land reform stretched to some 20 pages, longer than the anticipated text of the rest of the peace document combined.

Other major issues yet to be resolved when the two sides return to the bargaining table next month include drug trafficking and victim compensation, with the government continuing to insist that senior rebels accept jail terms as part of the peace deal.

Analysts applauded the announcement, saying it showed the peace process is further along and fundamentally different from the failed efforts of preceding decades.

"It is the first agreement reached in 40 years of conflict on the crucial issue of land," said Camilo Gonzalez, president of Colombia's independent Institute of Peace and Development Studies. "This confirms that this process is different from those of the past."

Some experts urged caution, though.

"All that is known is that the accord was announced, we know nothing of what it contains," said Alfredo Rangel, director of the Center for Security and Democracy at Bogota's Sergio Arboleda University. "This is a partial accord."

Negotiations began in October in Oslo, Norway, and have been held in Havana ever since, with the Cuban and Norwegian governments acting as guarantors. Santos, facing criticism of the negotiations and a re-election battle in 2014, has said a deal must be reached by November or his government will pull out.

The FARC has asked for 9 million hectares for what it calls "peasant reserves" with greater autonomy than the government is willing to offer. The government has only spoken publicly of returning stolen land to peasants and giving them title. Outgoing Agriculture Minister Juan Camilo Restrepo has rejected what he calls the FARC's idea of creating "independent republics" with the peasant reserves.

The Havana talks are the fourth attempt since the 1980s to bring peace to Colombia, which has been at war since the rebels took up arms in 1964. A U.S.-backed military buildup that began in 2000 has reduced the FARC's ranks to about 9,000 fighters and killed several top commanders, though the rebels insist they are still a potent force.