A month-long ceasefire by FARC guerrillas is holding in Colombia, officials said Tuesday, as the two sides continue to seek a negotiated end to a half-century of civil war.

"We haven't confirmed any offensive armed action by the FARC in that time that could be seen as a breach or violation of the ceasefire," the office of the Colombian government ombudsman.

The ombudsman's office said isolated incidents of violence have been reported in areas where the FARC is active, particularly in the south of the country, but the group's involvement could not be established.

The Revolutionary Armed Forces of Colombia on December 20 announced it would observe an indefinite, unilateral truce so long as its forces do not come under attack.

The Colombian government, for it part, has refused to agree to a ceasefire until a comprehensive peace agreement has been reached.

The leftist guerrillas have refrained from initiating new hostilities thus far.

The Bogota government and the FARC have been negotiating for more than two years in Havana to bring an end to the group's 50-year-old insurgency, but key issues remain unresolved, including disarmament and how any agreement should be ratified.

The peace talks being held in the Cuban capital Havana opened in November 2012.

Since 1964, more than 220,000 people have died in the fighting in Colombia.