About 30,000 civilians have had to leave the capital, Mogadishu, to escape the fighting
The US says it is seriously concerned by reports Eritrea is supplying arms to foreign fighters and Islamic hardliners fighting government forces in Somalia.
"This as a disturbing development," President Barack Obama's top official on Africa, Jonnie Carson, told the BBC.
Eritrea denies any involvement in arming or financing Islamist militants trying to overthrow the government.
Following a week of violence, 100 people are dead and 30,000 more have fled Somalia's capital, Mogadishu.
There have been a number of reports of foreign fighters, with possible links to al-Qaeda, fighting alongside hardline Islamists of al-Shabaab and Hisbul-Islam, said Mr Carson, the US Assistant Secretary of State for African Affairs.
AFRICAN UNION IN MOGADISHU
AU force in Somalia (Amisom) was mandated in January 2007
Supposed to be 8,000-strong but currently has only 4,300 troops
Comprised of soldiers from Uganda and Burundi
Sierra Leone has offered battalion, which would take force over 5,000
Restricted by security situation to operations in Mogadishu
"We're extremely worried about the reports."
"There seem to be fairly serious and creditable reports that al Shabaab does have, amongst its fighters, a number of individuals of South Asian and Chechen origin," said Mr Carson.
"This is a very disturbing situation and reflects the seriousness of the problem in Somalia."
Mr Carson also expressed concern about flights from Eritrea were carrying weapons and ammunition to Somalia to supply al-Shabaab.
"There have been numerous reports that the government of Eritrea has, in fact, been supplying weapons and munitions to al-Shabaab.
"These are reports that we do find credible," he said.
Somalia's President Sheikh Sharif Sheikh Ahmed has appealed to Islamist insurgents to negotiate as intermittent fighting continued for an eighth day in the capital, Mogadishu.
But his former ally and Islamist spiritual leader Sheikh Hassan Dahir Aweys has rejected his overture.
He told the BBC talks were not possible while African Union troops were in the city where they are guarding key sites.
On Friday, the United Nations Security Council unanimously approved a statement calling for opposition groups to end their offensive, renounce violence and join reconciliation efforts.
"There is no doubt, from sources overt and covert, that in the attempted coup of last weekend there was significant involvement of foreigners, some from this continent and others from outside this continent," the UN's envoy to Somalia, Ahmedou Ould-Abdalla said.
In recent days there have been suggestions that the Somali government might collapse, but Mr Carson ruled out deploying the 2,000 American troops currently stationed in neighbouring Djibouti into Somalia.
"This is an internal Somali matter," he told the BBC.
"I think that there would be no case of the US re-engaging on the ground with troops."
The Somali authorities control only one major road in Mogadishu, with the assistance of about 4,350 African Union troops.