Jendayi Frazer, US assistant secretary of state for African affairs, said on Thursday: "I think Eritrea is quite clearly attacking Ethiopia on another front. We have pretty clear evidence that's a fact and [they are] shipping arms into Somalia."
Eritrea has denied any involvement in Somalia, but a UN Security Council report in May said that it had sent weapons to the Islamic Courts Union in an attempt to frustrate Ethiopia with whom it fought a war between 1998 and 2000 and remains on bitter terms.
"The role of Eritrea in arming the ICU [Islamic Courts Union] and Ethiopia with its threats of intervention are unfortunate because it brings the conflict in Somalia to a regional level," Frazer said at a meeting in Nairobi.
Ethiopia is believed to have sent troops across the border into Somalia to support President Abdullah Yusuf's interim government against the Islamists' who have taken control of the capital, Mogadishu, and much of the south.
Meles Zenawi, the Ethiopian prime minister, vowed that his country would defend itself from the Islamic Courts Union which has threatened jihad against Ethiopian troops.
In an address to parliament, he said Islamist forces were about 15km from the border in central Somalia and warned they would be attacked by the army if they crossed it.
A suicide car bomber attempted
to kill the president in September
"The jihadists are planning to attack Ethiopia and they are endangering our national security," he said. "The Ethiopian government is ready to defend its territory from the Mogadishu Islamists".
At a meeting in Nairobi, aimed at salvaging peace talks, Yusuf appealed for international help in dealing with the Islamic Courts Union who he accused of planning to assassinate him and other government officials.
The president escaped a suicide car bomb attack in Baidoa, the seat of the interim government, on September 18.
"As a result of the investigation, our security forces have seized recent ICU [Islamic Courts Union] documents listing a considerable number of TFI [transitional government] leaders condemned as infidels and a target for immediate physical elimination," Yusuf said in a statement obtained by the Reuters news agency.
He cited a document that he said approved both his and Prime Minister Ali Mohamed Gedi's assassination.
Yusuf said it also ordered the killing of 16 other politicians and referred to the seizure of the port of Kismayo, which the Islamists took last month, and the positioning of fighters along the border along Somalia and Kenya.
The Islamic Courts Union has accused Ethiopia of carrying out the attack.
Yusuf also said he was pessimistic about Arab League-mediated peace talks with the group that are supposed to begin on October 30.
At the last round of talks in Khartoum, the two sides agreed in principle to create joint military forces and not make any further military moves. Both accuse the other of violating the deal.