Soldiers from the Comoros islands have clashed with forces on the island of Anjouan loyal to a renegade leader.
Hundreds of AU troops are due to lead an invasion of Anjouan
The fighting came ahead of a planned AU-led intervention in support of the Comoros president to oust Anjouan's self-declared president, Mohamed Bacar.
In fighting over the weekend, troops loyal to Mr Bacar had ambushed government troops on Anjouan's coast.
"The confrontation that followed led to the deaths of dozens of rebel forces," said the Comoran military.
Lt-Col Mohamed Salimou, the Comoran army's chief of staff, said two government soldiers had been injured during the second of two operations against the forces on Anjouan.
But Mr Bacar said Comoran President Ahmed Abdallah Sambi had no right to remove him from power.
"Sambi has violated the constitution of the union, and the constitution of Anjouan, and Comoran law many times, in the full view and knowledge of the representative of the African Union, without receiving any kind of reprimand," he said.
Language of violence
Since gaining independence from France in 1975, the Union of Comoros - an impoverished three-island archipelago off Africa's east coast - has suffered 19 coups or coup attempts.
Mr Bacar unilaterally declared himself president of semi-autonomous Anjouan last July after winning an election which the central authorities declared illegal.
Lt-Col Salimou said the renegade leader, a former French-trained gendarme, only understands the language of violence.
Several hundred AU troops from Tanzania, Senegal, and Sudan have arrived in Comoros to support an amphibious invasion by the country's federal National Development Army to regain control of Anjouan.
Last month a special summit of AU ministers agreed to send troops in support of Mr Sambi after mediation failed to end the months-long stand-off.
It is reported that the AU rejected a request from South African President Thabo Mbeki not to invade Anjouan.
Jose Francisco Madeira, the AU envoy to the Comoros, said efforts to persuade Mr Bacar to give up peacefully had failed, with the renegade leader refusing to hold fresh, AU-monitored elections or accept exile in another country.
With its history of political violence, the Comoros has teetered on the brink of disintegration since independence amid tensions between the semi-autonomous islands and the central government.