An ultimatum by the Colombian government to paramilitary fighters has pushed peace talks into a critical stage. Demobil- ization is the central theme.

BY STEVEN DUDLEY

BOGOTA - Peace talks between the Colombian government and right-wing paramilitary groups have entered a critical stage after the government ordered the capture of one of the top paramilitary leaders and demanded a demobilization of their estimated 15,000 fighters.

''If they don't start to demobilize immediately, then the peace process is not viable,'' the government's chief negotiator, Peace Commissioner Luis Camilo Restrepo, told local radio.

Restrepo's ultimatum Monday came just one day after the military issued an order for the capture of Juan Carlos Sierra-Ramírez, a paramilitary commander who was designated to the negotiating commission but has yet to appear publicly at the talks.

It's not clear whether the government moved against Sierra-Ramírez to turn up the pressure on the rest of the paramilitary negotiators, or as a gesture toward the United States, which has demanded his extradition. A Colombian court had approved his extradition.

He was indicted by a U.S. grand jury in 2002, along with paramilitary leaders Carlos Castaño and Salvatore Mancuso, for trafficking more than 17 tons of cocaine to the United States. Castaño disappeared in April under mysterious circumstances, and Mancuso is now one of the paramilitary's lead negotiators.

In May, the government promised to delay any legal action against the designated leaders of the United Self Defense Forces of Colombia, known as the AUC, if they agreed to remain in a 230-square-mile safe haven in the northern part of the country specially designated for the talks.

Paramilitary leaders in turn promised to negotiate toward a demobilization and disarmament of the estimated 15,000 fighters in the AUC, created in the 1980s by business owners, cattle ranchers and drug traffickers to protect themselves from leftist guerrillas.

While demobilization is the central theme of the negotiations, there are many hurdles to clear beforehand, including the question of U.S. extradition requests and local and international prosecution for human rights violations.

Despite their illegal activities, paramilitary leaders believe they should be rewarded for their anti-guerrilla efforts with an amnesty or reduced sentences.

The government of President Alvaro Uribe, facing pressure from the U.S. and European governments, publicly insists that the paramilitaries will have to pay for their crimes committed both in Colombia and abroad.

But there are hints that the negotiations include assurances that paramilitaries' top brass may not be extradited anytime soon.