A day after claiming to have captured an Ethiopian military officer in fierce weekend battles with a militia allied to Somalia's weak government, the union on Wednesday said at least 3,000 people had enlisted for combat.
Many new recruits have signed up in the past two days since the leader of the powerful Islamist movement called on residents to defend the country against Ethiopian troops allegedly in Somalia.
A senior official with the Supreme Council of Islamic Courts (SICS) told AFP in Mogadishu: "We have at least 3,000 young fighters who have now registered to fight the enemy of Allah."
The newcomers, including women, will join what the Islamists claim are tens of thousands of existing fighters who seized the capital, Mogadishu, in June. The Islamic courts now controls most of southern and central Somalia.
Sheikh Abdinur Farah, an Islamist commander who runs a jihad recruitment centre in Qoryoley, about 120km south of Mogadishu, said: "We have trained them to fight and that is religious obligation.
"Ethiopia has made clear its intention: that is a war against us. So we are calling an open war against Ethiopia and every young fighter is welcome to join the jihad against the Ethiopian invaders."
Ethiopia and the Somali government have repeatedly denied eyewitness accounts of Ethiopian soldiers in Somalia, although Addis Ababa has said several times it has sent trainers and advisers.
On Tuesday, the Islamists said they had seized an Ethiopian officer in clashes that killed at least 51 people north of the southern port of Kismayo.
Ethiopia has not yet responded to the alleged capture of the officer who was not identified by name or rank.
A day earlier, Sheikh Hassan Dahir Aweys, chief of the SICS and a religious leader designated a "terrorist" by the United States, urged Somalis to take up arms to repel the Ethiopian troops.