Nigeria's President Olusegun Obasanjo has weighed into a dispute over the role of Chad that has been holding up peace talks on the conflict in Sudan's Darfur region, one of the warring parties said on Monday. The argument over Chad's role as a mediator surfaced on the first day of the talks 10 days ago and has been hampering progress ever since. Obasanjo's move to resolve it signalled the issue has become a serious stumbling block for the entire negotiations, observers said. Obasanjo met delegates from Darfur's two main rebel groups overnight and listened to objections from one of them, the Justice and Equality Movement (JEM), about Chad's presence as a mediator at the peace talks in the Nigerian capital Abuja. "President Obasanjo noted that Chad had participated in the last four rounds of talks," Mahjoub Hussein, spokesman for larger rebel group the Sudan Liberation Army (SLA), told Reuters. "But since JEM has now raised objections to Chad's participation, he (Obasanjo) will raise these issues of contention with President Idriss Deby of Chad and get back to the movements as soon as possible," Hussein said. Obasanjo is chairman of the African Union (AU), the main mediator in the talks aimed at bringing peace to Darfur, where tens of thousands have died and more than two million have fled their homes into refugee camps inside Sudan and in Chad.