Sudanese government aircraft have attacked a site where rebel leaders in the Darfur region were planning to hold unity talks.
Ahmed Abdel Shafi, head of one of the Sudan Liberation Movement factions which did not make peace with the government last year, said the rebels had brought down one of two helicopter gunships.
A Sudanese armed forces spokesman said he was not aware of any attacks but a helicopter had gone missing on a reconnaissance mission after reporting a technical problem.
Protests are taking place around the world today to demand urgent intervention to end the conflict in Sudan's Darfur region.
Celebrities such as Bob Geldof and George Clooney are backing the Global Day for Darfur campaign, and have signed a statement accusing the international community of apathy.
The UN says some 200,000 people have died since the conflict began four years ago.
