In this recent but undated image released by the British Royal Marines Sunday March 25, 2007, marines from 539 Assault Squadron Royal Marines conduct operations on waterways in the Basra region of southern Iraq as part of Operation Troy. [AP]

TEHRAN, Iran - Iran said Tuesday the 15 British sailors and marines it detained last week are healthy, have been treated in a humane manner and that the only female sailor among them had been given privacy.

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"They are in completely good health. Rest assured that they have been treated with humanitarian and moral behavior," Mohammad Ali Hosseini, a spokesman for the Foreign Ministry, told The Associated Press.

Hosseini said the 26-year-old female sailor, Faye Turney, had complete privacy. "Definitely, all ethics have been observed," he said.

Hosseini would not say where the Britons were being kept and reiterated that their case is under investigation.

"The case should follow procedures," Hosseini said. "Media hyperbole will not help" speed resolution of the case.

Iran has said it is 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.

British Prime Minister Tony Blair said Tuesday he hopes diplomacy will win their release but is prepared to move to a "different phase" if not.

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 between Iran and Iraq has been disputed for centuries.

"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 replied: "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."

The exact path of the dividing line in the Shatt al-Arab waterway has long been disputed, in part because of shifting sands and mud in the waterway's extensive delta as it empties into the Persian Gulf.

The 125-mile waterway is formed by the confluence of the Tigris and Euphrates rivers and provides Iraq with its only access to the sea.

US officials said the crisis began when British sailors boarded an Indian-flagged commercial ship suspected of carrying smuggled cars through the waterway, which the Iranians call the Arvand River.

"It was an Indian-flagged vessel. It was suspected of being involved in automobile smuggling (into Iraq)," Cmdr. Kevin Aandahl, a spokesman for the US Fifth Fleet, told The Associated Press by telephone from fleet headquarters in Manama, Bahrain.

"It matched a profile of something we had told to be on the lookout for. It turned out to be not what we suspected and it was let go," he said.

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 US officials knew the GPS coordinates of the ship at the time the incident occurred, but would not release them publicly.

With the precise border line in dispute, the fate of the 15 Britons may depend on Iran's interpretation of their intent and whether they strayed across the frontier by accident.

On Saturday, Iran's military chief, Gen. Ali Reza Afshar had said the 15 confessed to "aggression into the Islamic Republic of Iran's waters."

But Deputy Foreign Minister Mehzi Mostafavi took a softer line Monday.

"It should become clear whether their entry was intentional or unintentional. After that is clarified, the necessary decision will be made," Mostafavi said.

Iran has refused to allow British officials to speak with the service members. But the official Iranian news agency said Iranian officials have told the British that their diplomats can see the 15 after the investigation is concluded.

There were fears in Britain that the fate of the 15 could get caught up in the political tensions between Tehran and the West, including the dispute over Iran's nuclear program and accusations of Iranian help to Shiite militants in Iraq.

In particular, there were worries Iran might seek to use the prisoners as leverage in trying to get the US to free at least five Iranians detained in Iraq for allegedly being part of a Revolutionary Guard force.

Mostafavi denied Iran was seeking a trade, but there were calls from elsewhere within Iran's leadership for the government to hold out for a swap.

A Web site run by Mohsen Rezaei, secretary of the influential Expediency Council and a former Revolutionary Guard commander, quoted an unidentified lawmaker as saying, "If Iranian diplomats in Iraq have no security, there's no reason why we should forgive and turn a blind eye to aggressors into Iranian territories."

Some members of the Iranian public also called for the British sailors and marines to be held and tried. Hundreds of Iranian students demonstrated near the coast to urge a tough stand.

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.

Iran's ambassador to Iraq, Hassan Kazemi Qomi, told The Associated Press in a telephone interview from Baghdad there was no connection between the capture and other disputes.

A 1975 treaty between Iran and Iraq set their border as running down the center of the Shatt al-Arab waterway, but Saddam Hussein canceled the treaty before invading Iran in 1980 and setting off a devastating eight-year war.

Iran claims the border runs along the deepest parts of the river.

On Tuesday, the US Navy began its largest demonstration of force in the Persian Gulf since the 2003 invasion of Iraq, with two aircraft carriers and backed by warplanes flying simulated attack maneuvers off the coast of Iran.

US Navy Cmdr. Kevin Aandahl said the US maneuvers were not organized in response to the capture of the British sailors - nor were they meant to threaten the Islamic Republic, whose navy operates in the same waters.