The federal president of the Comoros islands, Ahmed Sambi, has authorised military force to retake the island of Anjouan from rebels.
The AU is backing the federal government with troops
Mr Sambi said in a TV address Anjouan would be "liberated" in "coming hours or days" if the rebels did not give in.
A group of 1,500 African Union troops is supporting the Comoros federal government in its dispute with renegade Anjouan leader Mohamed Bacar.
It refused to recognise the re-election of Mr Bacar in June 2007.
Mr Sambi said on Monday: "I gave the order to the national army to join with the African Union forces to re-establish the republican legality in Anjouan.
"In the hours, or the days to come, Anjouan will be liberated by force, or, I dare to hope, the rebels will surrender themselves in order to avoid confrontation."
Five boats full of soldiers earlier left a port on the island of Moheli, a diplomat said, ahead of the operation to retake Anjouan.
Tanzanian and Sudanese forces are on board the boats, alongside Comoran soldiers.
Anjouan is reportedly tense but calm. Luggage trolleys are on the airport's runway to prevent any aircraft landing.
The island of Moheli is only about 50km from Anjouan.
Civilian flights to Anjouan and Moheli have been suspended, by the federal government.
AU troops took part in a beach-landing exercise on Sunday.
Each of the Comoros federation's three islands has its own president, but Mr Bacar's re-election was declared illegal by the central authorities.
In an interview on Thursday, he said he would not step down.
"I am still determined to defend Anjouan despite my concern that people are ready to come here and fire on the Anjouanese. But I am continuing with my preparations to defend Anjouan," he told AFP news agency.
He is estimated to have about 300 troops at his disposal.
There have been 19 coup attempts and several secessionist uprisings in the three-island archipelago since independence from France in 1975.