"If wishes were horses, the extremists in the Islamic Courts Union would have attacked Addis Ababa by now," Solomon Abede, a foreign ministry spokesman, said.
"Their declaration appealing to foreign Muslim fighters to help in war against Ethiopia proves their extremist behaviour."
Inda'ade made the call for foreign fighters after four days of clashes between Islamic opposition forces and pro-government troops.
Dozens have been killed in the fighting, with hundreds more wounded.
Ali Mohamed Gedi, prime minister in Somalia's largely powerless transitional government, said that foreign "terrorists" were already taking part in the fighting.
"Four thousand foreign fighters have participated in recent fighting around Dinsoor district and some of them have been killed," Gedi said.
Also on Saturday, the Islamic Courts said it has trained special forces to carry out guerrilla warfare against Ethiopian troops supporting the UN-backed government.
"Special forces who are highly trained in guerrilla warfare are now ready to attack Ethiopians, wherever they are in Somalia"
Sheik Ibrahim Shukri Abuu-Zeynab, a spokesman for the Islamic Courts
Tension across Horn of Africa
"Special forces who are highly trained in guerrilla warfare are now ready to attack Ethiopians, wherever they are in Somalia," said Sheik Ibrahim Shukri Abuu-Zeynab, a spokesman for the Islamic Courts.
Islamic forces opposed to Somalia's the transitional government have declared they want to bring the whole country under the rule of the Quran.
They have vowed to continue attacks to drive out troops from neighbouring Ethiopia, which is providing military support to the government.
Ismael Mohamoud Hurreh, the Somali foreign minister, told Al Jazeera that government was prepared for war.
"We are facing a group of people who are very dangerous and very radical, and the government is definitely on a war footing," he said
The United States has also accused the Islamic opposition of drawing support from al-Qaeda. The Islamic Courts have denied such a charge.
The most sustained combat in Somalia started late on Tuesday, the deadline the Islamic Courts had given Ethiopian troops protecting the government to leave the country or face war.
On Friday, officials said hundreds of people had been killed since Tuesday night.
Sporadic gunfire and shelling could be heard on Friday around Baidoa, the transitional government's only stronghold, but residents and officials said the fighting had subsided.
|Somali government forces have traded|
fire with Islamic Courts fighters [AFP]
Thousands of Somali civilians fled their homes in Baidoa after fighting erupted around the town.
The clashes threaten a wider conflict in the region. Ethiopia, which has one of the largest armies in the region, and its rival Eritrea could use Somalia as the ground for a proxy war.
While Ethiopia backs the internationally recognised government, Eritrea backs the Islamic Courts.
In an interview with Al Jazeera, Isaias Afewerke, the Eritrean president, denied reports that Eritrean forces are fighting alongside Islamic Courts fighters in Somalia. He said the reports were meant to justify what he called "Ethiopian occupation of Somalia."
"The Somalis do not need Eritrea to send 200 soldiers to protect them or strengthen their situation," Afewerki said. "We have asked many times about this, where is this Eritrean force in Somalia?"
In New York, Kofi Annan, UN secretary general, called on both sides to "cease the hostilities immediately and to resume the peace talks ... without delay and without any precondition" said his spokesman, Stephane Dujarric, in a statement released late Friday.
He also voiced "grave concern" over reports of the involvement of "foreign forces ... and he implores all involved to respect the sovereignty and territorial integrity of Somalia."
The UN issued a statement in the Kenyan capital, Nairobi, on Friday calling for an "immediate end" to the conflict.
The UN also said both sides were using increasing numbers of child soldiers. "This conflict will push the children of Somalia into further dire crisis," it said.