BOGOTA - (AP) -- The Colombian government warned Wednesday that peace negotiations with right-wing paramilitary groups could fall apart if the outlawed fighters fail to move into special zones as a first step toward disarmament.

''We are in a difficult moment, a critical moment,'' the government's peace commissioner, Luis Carlos Restrepo, told reporters. ``Without decisive change, this process may not be possible.''

Restrepo said he expected to meet with leaders of the umbrella paramilitary group, the United Self-Defense Forces of Colombia, or AUC, in upcoming days.

The AUC declared a unilateral cease-fire in December 2002 against leftist rebels fighting the Colombian government. Since then, however, paramilitary fighters have killed about 600 people, according to the United Nations, the Roman Catholic Church and human rights groups.

Restrepo has urged the militia to start gathering in zones of concentration dotted around the country, where the paramilitary groups would be put under the watch of international observers and likely receive the protection of the army.

The AUC has shown reluctance to move into the zones but hasn't refused outright. Some of its factions say they are not ready to abandon their strongholds because the army is not in a position to guarrantee security once they are gone.

The group rose up in the 1980s to combat the rebel insurgency, now entering its 40th year.

Under a pact signed with the government in July, the paramilitary group agreed to demobilize all of its 12,000 fighters by the end of 2005. About 1,000 have disarmed so far. Those concentrating in the special zones would eventually also put down their arms.