The leader of Darfur's main rebel group has been prevented from entering Chad while en route from Libya to Sudan.
Justice and Equality Movement (Jem) leader Khalil Ibrahim was told at Chad's airport to return to Libya.
A top Jem official told the BBC Chad is trying to "pressure" Jem into resuming peace talks with Sudan.
Jem has always had strong ties with Chad, but this has changed in recent months as relations between Chad and Sudan have improved.
The passports of Mr Ibrahim and other Jem members were confiscated and the Jem leader is currently at the airport in Chad's capital, N'Djamena.
The Chairman of Jem's Legislative Council Eltahir Adam Elfaki told the BBC's Focus on Africa programme that Chad's actions had not been a surprise.
"We have never been not suspicious," he said.
"We always suspect that sometimes deals that may be done behind the corridors would affect the relation [between Jem and Chad]."
In the past, Jem has regularly used Chad as a base for its troops and a transit point for its officials.
But in February, Chad's President Idriss Deby agreed with Sudan's President Omar al-Bashir to stop supporting rebels in each other's country.
Sudan has long accused Chad of backing rebels in Darfur, while in 2008, Chad accused the Sudanese of helping a rebel group which almost reached N'Djamena, before being beaten back.
It is unclear why Mr Ibrahim had travelled from Libya to Chad.
Jem had been told after the signing of the February agreement that it was no longer welcome in Chad.
And some observers believe that Chad's refusal to admit Mr Ibrahim is a sign that President Deby intends to respect his agreement with Sudan.
Jem signed a ceasefire with the Sudanese government in February but earlier this month left the peace talks being held in Qatar, claiming the government had launched new raids.
After Jem walked out of the talks, President Bashir asked Interpol to arrest Mr Ibrahim for planning an attack in Omdurman in 2008.
Mr Bashir is himself wanted by the International Criminal Court (ICC) for alleged war crimes committed in Darfur - charges he strongly denies.
Since the conflict in Darfur began in 2003, some 2.7 million people have fled their homes and the UN says about 300,000 more have died
The Sudanese government, however, says such figures are a massive exaggeration.
Another Darfur rebel group, the Liberation and Justice Movement (LJM), also signed a ceasefire with the government before the country's elections in April - the first multi-party polls in Sudan since 1986.
However a faction of the Sudan Liberation Army (SLA) led by Abdul Wahid is still holding out against the government and has refused to take part in the peace talks.
Darfur was relatively peaceful during the elections, which saw Mr Bashir re-elected.
But voting did not take place in much of the area because of the insecurity.