Militants have been battling pro-government forces for three years
Ethiopia has refused a request by Somalia for military support to fight insurgents, saying such an intervention would need an international mandate.
The Somali authorities have been battling Islamist insurgents who control much of the country.
The speaker of Somalia's parliament had earlier urged neighbouring countries to send troops within 24 hours.
Ethiopian troops helped topple an Islamist movement in Somalia in 2006, but were withdrawn earlier this year.
On Saturday Somali parliamentary Speaker Sheikh Aden Mohamed Nur urged neighbouring Kenya, Djibouti, Ethiopia and Yemen to intervene as fierce fighting continued for a second day in the capital Mogadishu.
But Ethiopian government spokesman Bereket Simon said that an international mandate was needed for such an intervention.
He added that the international community, not just Somalia's neighbours, should assist its transitional government.
Somalia has been without an effective government since 1991. Its UN-backed transitional government controls only parts of Mogadishu, but little of the rest of the country.
There are some 4,300 African Union troops deployed in Mogadishu, but they lack any mandate to pursue the insurgents.
Pro-government forces have been fighting radical Islamist guerrillas in the capital since 7 May.
On Friday, gunmen killed Mohamed Hussein Addow, an MP who represented the Karan district where fighting has been particularly intense in recent days.
It was the third killing of a high-profile public figure in as many days.
Somalia's security minister - an outspoken critic of the militant Islamist group al-Shabab - was killed in a suicide attack in the northern town of Beledweyne, and Mogadishu's police commander was also killed this week.
Militant groups including al-Shabab, which is accused of links to al-Qaeda, have been trying to topple Somalia's government for three years.
Some four million people in Somalia - or about one-third of the population - need food aid, according to aid agencies.