(Recasts with details of deal)

By Lamine Ghanmi and Alistair Thomson

DAKAR, March 13 (Reuters) - Sudanese President Omar Hassan al-Bashir and Chadian President Idriss Deby signed a peace agreement on Thursday designed to end cross-border rebel attacks in a region which includes Sudan's conflict-ravaged Darfur area.

The signing, witnessed by U.N. Secretary-General Ban Ki-moon and Organisation of the Islamic Conference (OIC) head Ekmeleddin Ihsanoglu, also aims to revive a string of past pacts that have failed to end fighting on both sides of the Chad-Sudan border.

"We solemnly pledge to ban the activities of all armed groups and to prevent the use of our respective territories to destabilise one or other of our states," said the agreement, brokered by Senegalese President Abdoulaye Wade.

The two oil-producing countries agreed to "normalise relations" and put a "definitive end" to their differences.

Foreign diplomats say Chadian rebels have regularly used the Darfur border region as a base from which to launch incursions into Chad. Sudan has in turn repeatedly accused Chad's government of backing Darfuri rebel groups.

The barren border areas of eastern Chad are home to half a million displaced, including Chadians uprooted by fighting and refugees from Darfur, where about 200,000 people have been killed in political and ethnic violence since 2003.

Instability on both sides of the border has hampered international efforts to deliver aid. It has also spilled into Central African Republic, worsening an insurgency there.

"CONTACT GROUP"

The latest pact foresees the formation of a "contact group" made up of foreign ministers from a handful of African countries, who will meet monthly to ensure the deal -- known as the Dakar agreement -- is implemented in good faith.

Deby and Bashir have met before to try to resolve their differences, which have brought the neighbours close to all-out war on a number of occasions.

They agreed in Dakar to commit themselves again to a string of past non-aggression pacts, brokered mostly by Libya but also by Saudi Arabia, which subsequently collapsed.

Senegal is hosting a two-day summit of the 57-nation OIC, a diverse body grouping a quarter of the world's population spread across Africa, Asia and the Middle East.

Wade, who has sought to mediate in several African crises, drafted the accord signed by Deby and Bashir in the hope that signing it during the summit, in the presence of so many international witnesses, would lend it some extra weight.

But sceptics abound.

Rebels from both Chad and Sudan's Darfur region, seen by many as fighting a proxy war for the feuding presidents, had already dismissed the pact, criticising it for failing to include them and saying it would not bring lasting peace.

Bashir, who accuses Deby of failing to respect previous deals, had questioned the usefulness of yet another accord.

Hours before the deal was signed, Chad -- which beat back a rebel attack on its capital only last month -- accused Sudan of sending rebels across the border into its territory.

Calling the attackers "mercenaries", the usual term it uses for Chadian rebels, Chad's government said the incursion occurred at Moudeina on its eastern border with Sudan's Darfur.

"I can assure you this is nonsense ... We have got no (Chadian) opposition inside Sudan. We closed our borders completely to these troops," Sudan's Minister of State for Foreign Relations, Al-Samani al-Wasiyla, told reporters.

There was no independent confirmation of an incursion.

Alex de Waal, an analyst on Sudan and Chad, said he did not believe either Deby or Bashir were interested in signing a serious peace deal. "If they do, it'll be purely for tactical reasons, to gain credit with the world community," he said.

(Additional reporting by Pascal Fletcher in Dakar and Andrew Heavens in Khartoum; Writing by Nick Tattersall; Editing by Ralph Gowling)

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