* UN chief Ban urges Chad, Sudan to cease fighting
* Arab League’s Moussa optimistic on Darfur peace outlook (Adds statement from U.N. in paragraphs 5-6)
By Andrew Heavens
EL FASHER, Sudan, May 18 (Reuters) - Sudan’s top official in North Darfur accused Chad on Monday of sending troops into his territory to fight alongside Darfur rebels, raising the stakes in the simmering tension between the two countries.
North Darfur governor Osman Kebir said Chadian forces had reinforced fighters from Darfur’s rebel Justice and Equality Movement (JEM) in an attack on the strategic town of Kornoi on Saturday.
Sudan accused Chad of carrying out three air strikes on its territory last week, calling the raids an "act of war."
However, this was the first occasion in recent times that it had said Chadian ground troops had breached its border.
U.N. Secretary-General Ban Ki-moon repeated his call for all parties to cease fighting, according to a statement from U.N. spokeswoman Marie Okabe.
"(Ban) underscores that there is no military solution to the situation in the sub-region and urges the governments of Chad and Sudan to refrain from any act that may lead to a further escalation of tensions," Okabe said.
Chad said on Sunday it had carried out the air raids, and fought near Sudan’s border, to destroy anti-Chadian insurgents it said were taking refuge inside Sudan. It has so far not commented on Saturday’s ground attack on Kornoi.
"Chadian aggression has reached the locality of Kornoi, a town near the Chadian border," said Kebir, speaking through a translator as he addressed a delegation of the leaders of the Arab League, the African Union Commission and the Organization of the Islamic Conference.
"Kornoi has been attacked by Chad forces with JEM. Our armed forces have stopped the aggression."
Chadian President Idriss Deby has ethnic links with JEM’s leader Khalil Ibrahim and many of his top commanders and Khartoum regularly accuses its neighbor of supporting the rebel force.
Amr Moussa, secretary-general of the Arab League, said he remained hopeful about the prospects for peace in Darfur, despite an increase in violence along the Chadian border.
"We are assured the government of Sudan is trying hard to mend fences with the government of Chad to move toward a low intensity situation," he told reporters at the end of his delegation’s one-day visit to El Fasher.
Jean Ping, chair of the African Union Commission, declined to comment on the reports of Chadian involvement in Sudan.
About 4.7 million people rely on humanitarian aid in Darfur, a conflict in which U.N. officials say as many as 300,000 people have died in almost six years of ethnic and political violence. Khartoum says 10,000 people have died. (Additional reporting by Megan Davies at the United Nations) (Editing by Andrew Dobbie and Todd Eastham)