LONDON - Britain hopes that diplomacy will win the release of 15 sailors and marines detained by Iran but is prepared to move to a "different phase" if not, Prime Minister Tony Blair said Tuesday.

The HMS Cornwall sails through the Gulf. Minister Tony Blair has warned Iran that the standoff over the capture of 15 sailors would enter a "different phase" if diplomatic efforts to secure their release failed. [Agencies]

Britain and the United States have said the sailors and marines were intercepted Friday just after they completed a search of a civilian vessel in the Iraqi part of the Shatt al-Arab waterway, where the border with Iran has historically been disputed.

"I hope we manage to get them (the Iranian government) to realize they have to release them," Blair said in an interview with GMTV. "If not, then this will move into a different phase."

Asked what that meant, Blair said: "Well, we will just have to see, but what they should understand is that we cannot have a situation where our servicemen and women are seized when actually they are in Iraqi waters under a UN mandate, patrolling perfectly rightly and in accordance with that mandate, and then effectively captured and taken to Iran."

Iran said Monday it was questioning the British sailors and marines to determine if their alleged entry into Iranian waters was "intentional or unintentional" before deciding what to do with them - the first sign it could be seeking a way out of the standoff.

A British reporter embedded with the Royal Navy reported in The Independent newspaper that the incident began when the sailors and marines approached a Japanese merchant ship suspected of smuggling second-hand cars into Iraq without paying tax.

That could not be immediately confirmed with US, Iraqi or British officials.

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Iran has refused to say where the captured Britons were being held or to allow British officials to speak with them, but assured the British ambassador to Tehran, Geoffrey Adams, that they were in good health.

Blair said his primary concern was the welfare of the British sailors and Marines.

"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."

Calls for the release of the Britons also came from the European Union, Iraq and the United States, under whose command the military search team was serving. Iraqi Foreign Minister Hoshyar Zebari said the personnel were seized in Iraqi waters and should be released.

A 1975 treaty between Iran and Iraq set their border as running down the center of the Shatt al-Arab, but Saddam Hussein canceled the treaty before invading Iran in 1980 and setting of a devastating war. Iran claims the border runs along the deepest parts of the river.