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's passport was seized at N'Djamena airport and he was told to return to Libya.
A Jem spokesperson says Chad is trying to force Jem to resume peace talks in Qatar with the Sudanese government.
In the past, Jem has regularly used Chad as a base for its troops and as a transit point for its officials.
Jem spokesperson Ahmed Hussein Adam said Mr Ibrahim and other Jem members were currently at N'Djamena airport.Rapprochement
Jem signed a ceasefire with the Sudanese government in February but left peace talks earlier this month, claiming the government had launched new raids.
After Jem walked out of the talks, Khartoum asked Interpol to arrest Mr Ibrahim for planning an attack in Omdurman in 2008.
- Jem One of the first rebel groups, leader Khalil Ibrahim is wanted by the Sudanese government
- SLA Main faction led by Abdul Wahid, lives in exile in Paris, not taking part in peace talks
- LJM Umbrella group formed in February, includes 10 smaller rebel groups
Sudan's President Omar al-Bashir is himself wanted by the International Criminal Court (ICC) for alleged war crimes committed in Darfur - charges he strongly denies.
Jem has always had strong ties with Chad's leadership.
But recently Chad's President Idriss Deby and Mr Bashir have said they want a full normalisation of relations, ending years of hostility between the two countries
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.