SIRTE, Libya — Darfur peace mediators vowed to press on with negotiations due to start today in Libya despite the decision by two main rebel groups to boycott the talks, saying time was running out for the Sudanese region torn by years of fighting.

Officials from the United Nations and the African Union plan to open the negotiations with a call for an immediate cease-fire commitment from all parties attending the talks.

However, yesterday’s announcement by the Justice and Equality Movement and SLA-Unity rebel groups that they will boycott the peace talks was a major blow to negotiations the U.N. hoped could reach a lasting peace agreement for Darfur.

The U.N.’s special envoy to Darfur and the joint mediator for the talks, Jan Eliasson, criticized the boycotting groups for not working to settle the conflict.

“I don’t see this as a failure for the negotiations, but as a failure for those who have not seized the opportunity to move toward peace,” a visibly upset Mr. Eliasson told reporters in Sirte, Libya, late yesterday.

“Anytime a significant movement figure is not present, it’s not a plus for the negotiations,” the AU envoy who is also mediating the talks, Salim Ahmed Salim, said.

The mediators insisted delegates from several splinter factions of JEM and SLA would take part in the Sirte talks. However, it was not clear whether these delegates had a mandate from their chiefs to negotiate a new peace.

The absence of the rebellion’s two main leaders means only a few minor splinter factions will attend the talks with the Sudanese government, which are geared at ending over four years of fighting that have killed more than 200,000 people.

U.N. Secretary-General Ban announced the Sirte peace talks in September, hoping to achieve a political settlement before the planned deployment to Darfur of a 26,000 strong joint AU-U.N. peacekeeping force in January.