A helicopter that crashed just off the Comoros island of Anjouan had intended to collect its renegade leader, a Comoros government official claims.
Mr Bacar declared himself the winner of polls last year
Abdul Bacar Soihir told the BBC the aircraft had come from the archipelago's French island of Mayotte.
He accused the French government of not supporting the Comorian bid to remove Mohamed Bacar from power in Anjouan.
His comments came as African Union troops joined Comoran troops on Moheli for a proposed invasion of Anjouan.
Each of the Comoros federation's three islands has its own president, but Mr Bacar's re-election last July was declared illegal by the central authorities.
Observers say that a divided Comoros would benefit the French because it would weaken any claim for Mayotte, the fourth island in the archipelago, which is under French rule.
The BBC's Jonny Hogg in the federation's capital, Moroni, says the French embassy there has refused to respond to the accusations.
The helicopter, with three passengers aboard, crashed a few hundred yards offshore from Anjouan's capital, Moulsamoudou, on Wednesday.
The French government has only said that the helicopter was not a military aircraft and had been on mission to track illegal fishing in between Anjouan and Mayotte.
They say it crashed because of mechanical failure although there have been reports it was shot down by fighters loyal to Mr Bacar.
"The helicopter came from Mayotte. Of course it was there to take Mohamed Bacar into exile," Mr Soihir, the cabinet chief, said.
"The reason Mr Bacar has resisted for so long against the government and the AU is that he has French supporters who are protecting him."
He said that Mr Bacar was being backed by unnamed private individuals on Mayotte.
"Verbally they support us but the French, the South Africans and the Americans are false friends to the Comoros," he said.
South Africa has been pushing for a diplomatic solution and fresh elections to end the crisis, but Mr Soihir insisted an operation to capture the island would go ahead.
"It is too late for dialogue. We've tried for a long time and it did not work, the only way now is to use force," he said.