Chadian rebels have dismissed a peace agreement between Chad and Sudan, saying they will continue their campaign to overthrow Chad's president.
Chadian rebels attacked the country's capital last month
The presidents of Chad and Sudan signed an accord on Thursday aimed at ending hostilities between the two countries.
A senior commander of the joint UN-Africa Union force in Darfur said the deal will not work unless rebels from both countries are included.
Meanwhile, the UN said it is returning many more refugees to South Sudan.
A spokesman for the Chadian rebel National Alliance said the non-aggression pact signed by Chad's President Idriss Deby and Sudan's President Omar al-Bashir did not concern them.
They want talks with Chad's president, said Ali Gadaye.
"If Deby doesn't want dialogue, then we're going to chase him out by force."
In their accord, the presidents of Chad and Sudan agreed to stop armed groups from using their respective territories to attack their neighbour.
In Sudan's West Darfur, bordering eastern Chad, the commander of the joint UN-AU peacekeeping force said the agreement will not have any impact unless the rebels are brought into negotiations.
As soon as rebels start fighting, Brig Gen Balla Kaita told the BBC, "nobody will know if it's between rebels and their government or is it going to be between the two countries".
The peace accord was mediated by Senegal's President Abdoulaye Wade and was signed on the sidelines of an Organisation of the Islamic Conference in Senegal's capital Dakar.
Just hours before it was signed, Chad accused Sudan of sending heavily armed columns of Chadian rebels across its border. There has been no independent confirmation of any crossing.
It is the sixth deal in five years and the war of words between the two sides is highly unlikely to end with the signing of the agreement, says the BBC's Will Ross in Dakar.
Chadian troops fought off an attempted coup last month in a fierce two-day battle in the capital N'Djamena.
At least 200,000 people have died and more than two million displaced in five years of conflict in Darfur.
Many refugees have crossed into Chad where a European Union force has recently deployed to protect them.
In South Sudan, the United Nations refugee agency said there is a growing desire among refugees from South Sudan to return home in time for a census next month.
The UN said 3,000 refugees a week had been repatriated to South Sudan from Uganda, Kenya and Ethiopia since January - up from 600 a week.
A national census in April is being held to prepare for Sudan's first democratic election in 23 years, due in 2009.
A referendum on whether South Sudan should secede is due two years later.