Chad has accused Sudan of sending "heavily armed columns" of fighters into its territory even as the two countries consider signing a peace accord.
The Chadian government anounced over national radio on Thursday that "these mercenaries crossed the border in the area of Moudeina", north of the town of Ade.
If true, the claim would be a repeat of an incursion that saw fighters in pickup vans ride into N'Djamena on February 2 and 3 in an attempt to overthrow Idriss Deby, the Chadian president.
Al-Samani al-Wasiyla, Sudan's foreign minister, said: "This is nonsense. I can assure you this is nonsense."
"We have got no [Chadian] opposition inside Sudan. We closed our borders completely to these troops."
Deby, whose government accuses Sudan of supporting Chadian fighters, was in Senegal's capital Dakar on Thursday, where he was set to sign a peace accord with Omar al-Bashir, his Sudanese counterpart.
Mediators hope the non-aggression pact will end years of hostility between the two countries that has brought them close to all-out war and aggravated the conflict in Darfur.
But al-Bashir, who accuses Deby of failing to respect previous deals to stop backing anti-Khartoum fighters, has questioned the usefulness of another accord on paper.
The signing of the accord was postponed on Wednesday after al-Bashir failed to show up, telling mediators he had a headache due to travel.
Ban Ki-moon, the UN secretary-general, waited in vain for al-Bashir at the Senegalese presidential palace in Dakar on Wednesday.
Ban and Deby left just before midnight, leaving Abdoulaye Wade, Senegal's president, to announce the reason for the meeting's collapse.
|Ban, left, and other leaders reportedly waited|
three hours for al-Bashir to show up [AFP]
Wade said: "I had a telephone call from President Bashir who told me: 'President Wade, I very much regret it, but I have travelled a lot and I have a headache. I cannot come in this state but I ask you to postpone the meeting until tomorrow morning."
For their part, armed groups in both Chad and Sudan's Darfur region - seen by many as fighting a proxy war respectively for Deby and al-Bashir - have already dismissed the planned accord.
The fighters criticised it for failing to include them and said it would not bring lasting peace.
"It's going nowhere. It's just a protocol, a ceremony," said Ali Ordjo Hemchi, a representative of the Chadian National Alliance, whose forces raided the country's capital N'Djamena last month.
"They can sign, but it's not going to produce anything."
Hemchi noted that at least five previous accords, brokered mostly by Libya but also by Saudi Arabia, had collapsed.
Armed groups in Sudan took a similar line.
Abdel Wahed Mohamed Ahmed al-Nur, chairman of the Sudan Liberation Army, said: "The Khartoum government has signed agreements with Chad ... agreements with the United Nations. And still our people are getting killed."
The area between Sudan and Chad has become a battleground for armed groups fighting both in Sudan's Darfur region and in eastern Chad.
Khartoum and N'Djamena accuse each other of supporting one or more of these groups.
About 200,000 people have been killed since 2003 in the Darfur conflict, which pits the Khartoum government's forces and allied militia against local fighters.