"Ethiopian defence forces were forced to enter into war to the protect the sovereignty of the nation and to blunt repeated attacks by Islamic Courts terrorists and anti-Ethiopian elements they are supporting," Meles said in a televised address.
"Our defence forces will leave as soon as they end their mission."
Berhan Hailu, the Ethiopia's information minister, said earlier his country's forces had targeted several fronts including Dinsoor, Bandiradley, Belet Weyne and the town of Buur Hakaba - close to the interim government's encircled base Baidoa.
Abdikarin Farah, Somalia's ambassador to Ethiopia, said that government forces had killed 500 Islamic Courts fighters, most of them Eritreans, in the last two days. He said that 280 others had been taken prisoner, some of them from Pakistan, Afghanistan and Sudan.
The Islamic Courts say they have killed hundreds of pro-government troops, but Farah claimed that just 10 soldiers had died and 13 wounded.
Aid agencies say that dozens have died in the fighting.
Ethiopia has previously denied that its forces were fighting in Somalia, saying it had sent only military advisers.
Sheikh Mahmud Ibrahim Suley, an Islamic Courts leader, said the Ethiopians had used MiG warplanes and helicopters against their positions.
"[The Somali government] will defend the people it is responsible for and Somali sovereignty"
Ali Mohamed Gedi, Somali PM
"Today the war is being fought by land and air," he said, adding that the group's fighters had destroyed five Ethiopian tanks.
Ali Ahmed Jama, the Somali government information minister, said: "Fighting is going on from one part of the country to the other. The Islamic Courts ... will lose in this fighting."
Witnesses said that the Ethiopian combat aircraft bombed towns in central Somalia.
"I saw five military aircraft flying over Belet Weyne," said Ahmed Atto, a freelance reporter in the area.
One source from the town of Belet Weyne said: "The planes hit a centre where Islamic officials in the region have been enrolling volunteers who wanted to join the war."
Another witness said strikes were attacking the roads and defences of the Islamic militia.
"Now there is a full-blown war," said one fighter close to the semi-autonomous Puntland region, north of Baidoa.
Somalia is seen as a potential proxy battleground for Ethiopia, which has one of the largest armies in the region, and its rival Eritrea, which backs the Islamic Courts.
The Islamic Courts Union has vowed to drive out Ethiopian troops that are providing military support to Somalia's UN-backed government.
Ibrahim Hassan Adow, the group's foreign affairs chief, said the courts will not negotiate with the government because the administration is the "puppet of another foreign country".
|The fighting may ignite a regional conflict [AFP]|
He also denounced the US, saying Washington should be calling on Ethiopia to withdraw its troops.
Ali Mohamed Gedi, the Somali prime minister, has vowed that his government will "defend the people it is responsible for and Somali sovereignty" and said the Islamic Courts fighters should return to negotiations.
Several rounds of talks, mediated by the Arab League, have failed to produce any lasting effect.
"They will be responsible for any consequences that may result from rejecting our call," Gedi said.