Tony Blair has warned of a "different phase" if talks fail to secure the release of the 15 British service personnel held in Iran.

Eight sailors and seven Marines from the Type 22 frigate HMS Cornwall were carrying out a routine search of a vessel which they suspected of smuggling.

They were held at gunpoint by Iranian forces in the mouth of the Shatt al Arab waterway which divides Iran and Iraq.

Mr Blair said: "Our first concern is for their welfare and to get them released as quickly as possible. What we are trying to do at the moment is to pursue this through the diplomatic channels and make the Iranian government understand these people have to be released and that there is absolutely no justification whatever for holding them.

"I hope we manage to get them to realise they have to release them. If not, then this will move into a different phase.

"At the moment, what we are trying to do is to make sure that that diplomatic initiative works."

Mr Blair did not explain what he meant by a "different phase".

Iranian 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.

In a statement released 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."

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