Three foreigners arrested in Kabul this week were on a freelance counter-terrorism mission, the Afghan government says.
Idema may have been acting beyond the control of US troops
They include at least one US national, Jonathan K Idema, a purported former US special services soldier who says he helped fight the Taleban in 2001.
The men were detained for illegally holding Afghans in a private prison.
The Interior Minister, Ali Ahmad Jalali, said their activities had no legal standing.
US officials say Mr Idema had tried to pass himself off as an American government or military official.
The BBC's Andrew North in Kabul says this is the first official confirmation that Mr Idema and his group may have been acting beyond the control of US forces or the Afghan government in hunting alleged members of al Qaeda and the Taleban.
The interior minister said Mr Idema and his colleagues had arrested eight people from across Afghanistan and imprisoned them.
"They apparently said that their aims were to act against those carrying out terrorist attacks," Mr Jalali told journalists in Kabul.
"But they did not have a legal relationship with anyone and the United States was also chasing them - they are actually rebels," he said.
Earlier this week the US embassy warned journalists in Kabul about Mr Idema who says he helped anti-Taleban rebels fight the hard-line Islamic regime back in 2001.
"The public should be aware that Idema does not represent the American government and we do not employ him," a US statement said.
Security sources have told the BBC that the US military circulated warning notices about Mr Idema some time ago, describing him as armed and dangerous and accusing him of interfering with military operations in Afghanistan.
Mr Idema has offered his services to the western media as an expert on fighting the Taleban and as Qaeda and has also featured in a bestselling book, The Hunt for Osama bin Laden.
The three men were arrested by Afghan police and intelligence officials in a district of Kabul on Monday.
Reports on Wednesday said all three were American. But the nationalities of the other two are not clear.