As the Islamists and weak Somali government gird for war after the collapse of peace talks, the US embassies in Nairobi and Addis Ababa warned Americans "to use extreme" caution while in Kenya and Ethiopia, citing the threat.
They said they had information about "reports of terrorist threats emanating from extremist elements within Somalia, which target Kenya, Ethiopia, and other surrounding countries."
"These threats specifically mention the execution of suicide explosions in prominent landmarks within Kenya and Ethiopia," the embassies said in notices sent to US citizens in the two countries.
"American citizens are advised to remain vigilant and to use extreme caution when frequenting prominent public places," said the warnings, issued as Nairobi prepares to host some 6,000 foreign visitors at a UN climate change conference.
An official with the US embassy in Nairobi said the warning was prompted by postings on Somali websites purported to come from Sheikh Hassan Dahir Aweys, the Islamists' supreme leader.
The statements attributed to Aweys, a cleric designated a "terrorist" by the United States for alleged ties with al Qaeda, authorise attacks in Kenya and Ethiopia, the official said.
"They say that he condones suicide operations," said the official, speaking to AFP on condition of anonymity, adding that Washington had not verified the authenticity of the threat but was taking it seriously.
Another embassy official said no specific targets were identified for attack by the postings that referred to public areas where large numbers of people gather in Kenya, Ethiopia and unnamed other countries in the region.
Islamist officials in the Somali capital Mogadishu referred questions about the matter to Aweys, who could not immediately be reached for comment.
Aweys has in the past denied any connection to terrorism and rejected US accusations that the Islamists are harbouring al Qaeda suspects wanted over the deadly 1998 bombings of its embassies in Kenya and Tanzania.
The US warning comes as the Islamists have declared "jihad", or holy war, on neighbouring Ethiopia for allegedly sending thousands of troops to Somalia to back the government and accused Kenya of siding with Ethiopia.
Both Kenya and Ethiopia favour the deployment of regional peacekeepers -- vehemently opposed by the Islamists -- to help the government, whose limited authority is increasingly threatened.
The deteriorating situation in Somalia has sparked fears of an all-out war that could engulf the Horn of Africa in wider conflict, possibly drawing in Ethiopia and its arch-foe neighbour Eritrea, accused of backing the Islamists.
Earlier this year, a covert US program to support Somali warlords battling the Islamists for control of Mogadishu failed disastrously when the capital fell after months of fierce fighting.
The Islamists have since rapidly expanded their territory to include most of southern and central Somalia, where they have imposed strict Sharia law, fuelling concern of a Taliban-style take-over of the lawless country.
In Washington, the US State Department said it was working with Arab states in a bid to convince the Islamists to return to negotiations with the interim government and avoid wider unrest.
"We're trying to work with the Arab League, which sponsored the talks in Khartoum, to try to get the parties back together," department spokesman Sean McCormack said.
"The Islamic courts have placed some conditions on going back to the talks, we don't think that those are appropriate," he said.
The US has also been pressing Somalia's neighbours, notably Ethiopia and Eritrea, not to escalate tension in the country.
"We would call upon Somalia's neighbours to play a positive role in Somalia and not use the situation in Somalia as a way to further destabilise the situation there," McCormack said.

Copyright AFP (Agence France-Presse), 2006