The missing sailors and marines were captured last Friday after boarding a dhow carrying suspicious cargo off the coast of Iraq.
Although Britain insists its forces were not doing anything wrong, the Iranians have condemned the move as an act of “aggression” and threatened prosecution.
Deputy Foreign Minister Mehzi Mostafavi said the captives were being interrogated but insisted they were not taken as pawns to be used in exchange for five Iranians held in Iraq.
Releasing a statement through state television, he said: “Iran has enough evidence to prove that the British forces personnel were detained in Iranian waters.
“It should become clear whether their entry was intentional or unintentional. After that is clarified, the necessary decision will be made.”
He gave no further information as to what will happen to them. The Iranians held in Iraq are suspected of helping to provide arms and money to insurgents.
Speaking in the House of Commons, Mr Browne said his thoughts were with those detained, their friends in theatre and families in the UK. He added: “I do not intend to comment further on this issue other than to say we are doing everything possible to secure their release.”
The British group seized last week was held at gunpoint by Iranian forces in the mouth of the Shatt al Arab waterway which divides Iran and Iraq.
The eight sailors and seven Marines – including one woman – from the Type 22 frigate HMS Cornwall were carrying out a routine search of a vessel which they suspected of smuggling.
