Western leaders hailed a "breakthrough" agreement with Sudan brokered by outgoing U.N. chief Kofi Annan, but significant differences remained over the shape of a peacekeeping force.

Shortly after Annan declared the outcome of a high level meeting in Addis Ababa, Chad announced plans to send troops to help Central African Republic confront cross-border rebels in what it said was a spreading regional war waged by Sudan.

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The announcement signaled an escalation of the three-year Darfur conflict, which has spilled an explosive mixture of refugees, rebels, militia and bandits over Sudan's western borders into Chad and Central African Republic.

"It is agreed in principle that, pending clarification of the size of the force, we should be able to take it forward," Annan said late on Thursday at the end of the Addis Ababa meeting.

"The troops should be sourced from Africa as far as possible and the command and control structure would be provided by the U.N.," the U.N. Secretary-General said, adding the force would consist of 17,000 troops and 3,000 police.

But it was clear there was some way to go before the force could be deployed, with Khartoum still not agreeing on either its size or any U.N.-African Union joint command.

Annan's remarks on the command and control structure were challenged by Sudanese Foreign Minister Lam Akol who told Reuters: "We have not agreed to this. We agreed to accept the U.N. support to the African Union troops."

Sudan's U.N. ambassador Abdalmahmood Abdalhaleem Mohamad told reporters after the Addis meeting the force would have an African commander and have U.N. logistical support.