Talks in Sudan between Somalia's weak interim government and Islamists have been delayed.
An Islamist delegation has flown to Khartoum but they say they will not take part in the talks unless Ethiopian troops leave Somalia.
Ethiopia denies having a fighting force in Somalia, but says it has hundreds of military trainers with the government.
Some fear a regional conflict breaking out in Somalia, amid reports that Eritrea backs the Islamists.
The AP news agency says it has seen a confidential UN report, in which diplomats say 2,000 fully armed troops from Ethiopia's rival, Eritrea, are helping the Union of Islamic Courts (UIC).
"Unless they [Ethiopians] leave Somali territory there is no point in us attending the meeting"
The UN report also says there are between 6-8,000 Ethiopian troops, according to AP.
On Sunday, the authorities in Yemen, just across the Red Sea from Somalia, said they had arrested several foreigners, including three Australians and one Dane, on charges of trying to smuggle weapons to Somalia.
The UN has imposed an arms ban on Somalia but weapons are easily available in a market in Mogadishu.
'Internal disputes'
The government delegation has arrived in Khartoum but the two sides are to hold separate meetings with mediators before possibly holding direct talks in the coming days.
An unnamed Arab League official told the AFP news agency that the talks had been "delayed because of internal disputes among the transitional federal government".
"We are here for the dialogue," government delegate and former Foreign Minister Abdullahi Sheikh Ismail told the Reuters news agency
Before boarding a plane for Khartoum, UIC foreign relations chief Ibrahim Hussein Adow said: "Unless they [Ethiopians] leave Somali territory there is no point in us attending the meeting."
He also objects to Kenya chairing the talks.
These are the third round of talks the Arab League has organised in Khartoum.
The two sides have previously agreed a ceasefire but the UIC has continued to gain ground, while pro-government forces have twice seized a key town before withdrawing.
The UIC has rapidly taken control of most of southern Somalia since seizing the capital, Mogadishu, in June.
The government only controls the territory around Baidoa.
Somalia has been in the grip of warlords and militias for years and has not had a functioning national government since 1991.
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