Prime Minister Tony Blair warned on Tuesday that his government could be forced to make public its proof that a captured British navy crew was in Iraqi waters when seized by Iran, saying the standoff may move into a "different phase" if diplomacy fails to win their release.

Britain has said the crew was detained Friday after completing a search of a civilian vessel in the Iraqi part of the Shatt al-Arab waterway, where the border with Iran is disputed.

Blair's office said Tuesday that the sailors were well inside Iraqi territorial waters when they were seized. "This is not a case of marginally being in Iraqi waters, but in Iraqi waters," Blair's official spokesman said, on condition of anonymity in line with government rules.

"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."

Blair's spokesman said later the prime minister was not hinting either at the possible expulsion of Iranian diplomats or military action, but instead suggesting Britain would adopt a more robust approach to negotiations.

If there is no swift release of the sailors, Blair's office said, diplomats would likely make public evidence that they claim proves the Britons were seized in Iraqi — not Iranian — waters.

Foreign Secretary Margaret Beckett called the Iranian Foreign Minister for the second time Tuesday and demanded the speedy release of the British sailors in "very robust terms," the Foreign Office said. Beckett was in Turkey on an official visit, which she cut short Tuesday to return to London for a statement to Parliament on Wednesday.

Blair's spokesman did not reveal whether Britain had set a deadline to release the sailors, but said current negotiations were not an "indefinite approach."

He refused to reveal if satellite images or GPS coordinates had been shown to Iranian authorities and declined to reveal which territorial boundaries in the disputed Shatt Al-Arab Britain recognizes.

Releasing evidence that the British sailors were in Iraqi waters would have "an upside and a downside," Blair's spokesman said, acknowledging the coordinates could show Iranian ships had strayed into Iraqi territory and provoke a diplomatic row between the neighbors.

Blair said his primary concern was the welfare of the navy personnel.

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Iran said Tuesday crew were healthy and being treated well and that Faye Turney, 26, the only woman among the crew, had been given privacy. In talks with the Iranian Foreign Minister, Beckett demanded immediate consular access so that British diplomats could make their own assessment.

Britain's ambassador to Iran, Geoffrey Adams, also held talks with Iranian diplomats Tuesday, as a U.S. official gave the first detailed account of the incident that sparked the seizure of the Britons.

The British crew had boarded an Indian-flagged commercial ship when they were seized Friday by Iranian naval forces.

"It was an Indian-flagged vessel. It was suspected of being involved in automobile smuggling (into Iraq)," said Cmdr. Kevin Aandahl, a spokesman for the U.S. 5th Fleet, in a telephone interview with The Associated Press from fleet headquarters in Manama, Bahrain.

Aandahl said the captain of the Indian ship had provided a statement that his vessel was in Iraqi waters at the time it was stopped by the British. He said U.S. officials knew the GPS coordinates of the ship at the time the incident occurred, but would not release them publicly.

Boundaries between Iraqi and Iranian parts of the Shatt al-Arab waterway have long been disputed. "They have never come to an agreement as to who actually controls those waters," said Sara Russell, an instructor at the Maritime Institute of Old Dominion University, in Norfolk, Virginia.

Asked whether he thought Iran was retaliating for the arrest of five Iranians by U.S. forces in Iraq, Blair said that should have absolutely no bearing at all on the fate of the sailors.

"Any Iranian forces who are inside Iraq are breaching the U.N. mandate and undermining the democratically elected government of Iraq, so they have got no cause to be there at all," Blair said.

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