Confusion surrounds the fate of Saifallah Benahssine – the head of Tunisian’s Islamist militants.
Tunisian state media has claimed that he was captured on Monday.
The report said he had been detained in neighbouring Libya by US special forces and Libyan troops.
The Americans denied they had any role in any such operation.
The Libyan government has refused all comment on the matter.
Benahssine’s organisation, the al Qaeda-linked Ansar al Sharia, insists he is still at liberty.
Tunisia’s state news agency TAP quoted a security source as saying Benahssine has been held in the Libyan city of Misrata.
Ansar al Sharia is one of the hardline jihadist groups to emerge after the Tunisia’s revolt against its autocratic leader three years ago.
Benahssine is on the wanted list for allegedly inciting an attack on the US embassy in the Tunisian capital last year and links to political assassinations there.
Also known as Abu Iyadh, he is a former fighter in Afghanistan.
Washington’s denial is no surprise. Any US involvement in an operation on Libyan soil would be highly sensitive. Libyan Islamists were angered when US forces captured a top al Qaeda suspect in Tripoli in October.