* Up to 300 foreign Islamist fighters in Somalia
* An Afghani national killed in Mogadishu
* At least 16 die in clashes in central Somalia
(Adds AU statement, new fighting)
By Frank Nyakairu
NAIROBI, May 15 Hundreds of foreigners from Africa and outside the continent are battling Somalia's western-backed government in the worst clashes for months, the U.N. special envoy to the Horn of Africa nation said on Friday.
Intelligence agencies are worried that Somalia -- with its porous borders and coastline, al Qaeda-linked Islamist groups and weak government -- may become a beach-head for militants trying extend to their influence in the region and beyond.
Some observers play down that risk, saying most Somalis follow a moderate form of Islam and have a deep suspicion of foreigners and the strict interpretation of Islamic law espoused by groups such as al Shabaab.
"There is no doubt from many sources, covert or overt, that there is a significant number of foreign fighters in Somalia from within the continent and outside," U.N. envoy Ahmedou Ould-Abdallah told reporters in Kenya's capital.
"I have seen figures from U.N. Security Council documents submitted by the U.S. where they are estimated to be between 280 and 300," he said.
Fighting between al Shabaab militants -- who admit to having foreigners in their ranks -- and pro-government fighters has killed at least 139 people and sent some 27,000 fleeing the pock-marked, seaside capital since late last week.
At least 16 people were killed in central Somalia on Friday in clashes between Shabaab soldiers and fighters from the moderate Islamist group Ahlu Sunna Waljamaca in Mahas and Wabho.
"We want to sweep al Shabaab completely. Sporadic fighting goes on in the outskirts of these two towns," said Sheikh Abdullahi Sheikh Abu Yusuf, spokesman of Ahlu Sunna.
The African Union (AU) envoy to Somalia told reporters that a foreign fighter was killed in Mogadishu on Friday.
"We are aware that one of them from Afghanistan has been killed in the fighting," Nicolas Bwakira said.
"There are elements of al Qaeda within al Shabaab, and it would be unacceptable for al Qaeda and al Shabaab to take power in Somalia, because they are a group of war criminals," he said.
Analysts say foreigners have been training insurgents in explosives and tactics, while weapons such as landmines, grenade launchers and rockets have been flown and shipped into Somalia.
A security analyst said roadside bombs targeting AU peacekeepers have become more sophisticated in recent months.
Hardline opposition leader Sheikh Hassan Dahir Aweys told Reuters on Thursday there were some foreigners in Somalia, but that those numbers had been embellished [ID:nLE392447].
Aweys -- who, Washington says, has links to al Qaeda -- also accused U.N. envoy Ould-Abdallah of "destroying" Somalia through his support of the transitional government, and said there would be no talks until the AU force left.
Since early 2007, an insurgency against the government has killed 17,700 people and wounded almost 30,000 others, according to a local rights group. (Additional reporting by Abdi Sheikh, Abdi Guled and Ibrahim Mohamed in Mogadishu; Writing by Jack Kimball; Editing by Giles Elgood)