A spokesman for the Hawai clan said: "I call upon the Somali people, wherever it exists, to unity in the fight against the Ethiopians. The war is not between Ethiopia and our tribe, it is between Ethiopia and all Somali people."
The clan's call to arms comes as Ethiopian troops engage in daily - and increasingly heavy - battles against Somalis opposed to their presence and to the interim government.
Diplomats from countries neighbouring Somalia have met in Kenya on Friday to discuss the role and cost of several thousand African Union troops stationed in Somalia.
They are meeting in the Kenyan capital under the auspices of East Africa's seven-member inter-governmental authority on development.
Kenya, Uganda, Djibouti, Ethiopia, Eritrea, Sudan and Somalia make up the east African bloc.
They were expected to push for more support for AU peacekeepers and national reconciliation in Somalia.
However, the meeting of foreign ministers appeared to have quickly became a forum for the festering feud between Ethiopia and Eritrea.
The countries are still bitter rivals over their 1998 to 2000 border war and locked in what many see as a proxy war in Somalia.
In a separate development, Sheikh Sharif Sheikh Ahmed, the leader of the Islamic Courts fighters, has met with Isaias Afwerki, Eritrea's president, in Asmara, the Eritrean capital.
It is the first time that an African leader has met with the group since the fighters were driven out of Somalia by Ethiopian soldiers, backed by US air strikes.
Ahmed is in Asmara to get support from its government, who has been its ally in the past against Ethiopia.
Hussien Aidid, deputy prime minister of Somalia, is also in the capital.
Hussien Aidid, Somalia's deputy prime minister,
says Ethiopia plans genocide against his people
He accuses Ethiopia of planning genocide against his people and allowing Ethiopian troops to be stationed illegally in the country.
Aidid said: "I came to explain to the Eritrean leadership the genocide the Somali people are facing as cities are being bombed and children are being killed."