Colombia's FARC rebels have announced a two-month unilateral ceasefire, the first truce in more than a decade.

President Juan Manuel Santos' government, however, has said it would not cease military operations until a final peace deal was signed with the Andean country's largest rebel group and even vowed to step up the offensive.

The FARC said it would halt all offensive military operations and acts of sabotage against infrastructure beginning at midnight tonight and running through 20 January.

"This policy decision of the FARC is a contribution made to strengthen the climate of understanding necessary so that the parties that are starting the dialogue achieve the purpose desired by all Colombians," FARC lead negotiator Ivan Marquez said as he arrived for talks in Havana.

The conflict has dragged on for nearly half a century, taking thousands of lives and displacing millions in Latin America's longest running insurgency.

Failure would mean years more of fighting and further blight on the reputation of a country eager for more foreign investment and regional clout, yet unable to resolve its most serious domestic problem.

Delegations for the government and the FARC arrived in black luxury cars at Havana's convention centre where they will meet almost daily until the talks end.

The complex is located in Cubanacan, Havana's plushest neighbourhood, filled with palatial houses that once belonged to the city's elite, virtually all of whom fled Cuba after the 1959 revolution that transformed the island into what is now one of the world's last communist states.

New optimism

The conflict proved to be intractable in three previous peace processes, but both the Colombian government and the FARC, or Revolutionary Armed Forces of Colombia, have expressed optimism that this time might be different.

Mr Santos wants an agreement within nine months, but the two sides face plenty of thorny issues in their five-point agenda, which will begin with rural development.

The other four points are the political and legal future of the rebels, a definitive end to the conflict, the problem of drug trafficking and compensation for war victims.

The conflict dates back to 1964 when the FARC emerged as a communist agrarian movement intent on overturning Colombia's long history of social inequality.

The group has been weakened by the 2002 US-backed military offensive that has reduced its numbers to about 8,000 and forced them into remote mountain and jungle strongholds.

But it still has the strength to launch attacks that Mr Santos wants ended so the country can grow its economy, boosted in recent years by fast-growing oil and mining sectors.

The FARC has sustained itself by cocaine trafficking, kidnapping, ransom and "war taxes" charged within the territories it controls.

Its leaders deny involvement in the drug trade and renounced kidnappings earlier this year, but the United States and European Union consider it a terrorist organisation.

Mr Marquez, a member of the FARC's secretariat, will lead a delegation of about 30 people at the talks, which formally began last month in Norway.

He has said the talks are likely to take longer than Mr Santos wants because so many complicated issues must be resolved.

In Norway, he called for ousting foreign companies fuelling Colombia's oil and mining boom.

However, lead government negotiator Humberto de la Calle said the talks must stick to the agenda if they are to be successful.

The agreed upon topics already hold numerous potential stumbling blocks, among them land reform, decisions on which FARC leaders will be allowed to participate in politics, and who must go to jail for the group's crimes and involvement in the drug trade.

Norway is a guarantor of the process, along with Cuba.

Venezuela and Chile also will have representatives at the talks.