Man held in Italy jail for 18 years killed in Somalia blast
UK rights group Reprieve says it has evidence that the base lies underneath the presidential compound, and that some inmates are as young as 14.
The group says one man was taken there from Kenya and held for 18 months without seeing lawyers – or daylight.
US officials have not yet commented on the claims.
Prime Minister Abdiweli Mohammed Ali told the BBC that he rejected the reports of a detention centre.
“I do not have that information and that information does not exist,” he said.
However, he did admit that the US was helping his authority to “improve the security situation in the country”.
Islamist militants from the al-Shabab group – which is known to have links with al-Qaeda – control much of southern and central Somalia.
The UN-backed transitional government is largely confined to the capital.
On Thursday, Mr Ali said he was open to talks with al-Shabab.
Claira Gutteridge from Reprieve told the BBC’s Network Africa programme that she had evidence from “multiple, concurrent sources”.?She said one man, Ahmed, had been abducted from the streets of Nairobi 18 months ago an flown to Mogadishu, where he has been kept ever since.
“It’s an underground prison in the compound of the presidential palace,” she said.
“The guards are Somali but it appears American personnel have access to the prisoners whenever they like.”
She said it was not clear how many people were held there, but they were mostly Somali and included children as young as 14.
However there were at least three Kenyans and one who had a Western passport, she said.
She added that it was unclear whether American agents were abducting and transferring – or “rendering” – suspects to Mogadishu, or getting Kenyans and Somalis to do their “dirty work”.
In July, the US Nation magazine reported the existence of a detention centre.
The US has carried out several air raids against al-Shabab and alleged al-Qaeda operatives in Somalia in recent years.
It has a military base in neighbouring Djibouti.
No group has claimed responsibility for his murder, but such incidents bear all the hallmarks of past attacks in Somalia by Al-Shabaab militants.