LONDON, England (CNN) -- British Prime Minister Tony Blair has said his country is prepared to move to a "different phase" if negotiations fail to free 15 sailors and marines being held by Iran.
Iran's Revolutionary Guards captured the group Friday, while it was conducting what Britain called an inspection of a merchant vessel near the mouth of the Shatt al-Arab, at the northern end of the Persian Gulf.
Britain said the Royal Navy crew were seized in the Iraqi part of the waterway, where the border with Iran is disputed. Iran however says the group was in Iranian waters.
In an interview on GMTV on Tuesday Blair said: "I hope we manage to get them (the Iranian government) to realize they have to release them. 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 U.N. mandate, patrolling perfectly rightly and in accordance with that mandate, and then effectively captured and taken to Iran."
Blair's spokesman said later the prime minister did not mean Iranian diplomats would be expelled or military action was likely.
His office did say though that officials may have to make public evidence proving the Britons were seized in Iraqi -- not Iranian -- waters, if there is no swift release of the sailors.
"We want to resolve this quickly without having a public confrontation with them, but as the prime minister said we want to resolve it. If we cannot resolve it quickly, then maybe we have to be more explicit," said the spokesman.
Earlier Tuesday, Iran said the 15 sailors and marines were healthy and being treated in a humane fashion.
"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 on Tuesday.
Hosseini said the one woman in the group had complete privacy. "Definitely all ethics have been observed," he said.
The official declined to say where the marines were being held and repeated that their case was under investigation.
"The case should follow procedures," Hosseini said. "Media hyperbole will not help (speed things up)."
Meanwhile on Tuesday the U.S. Navy started its largest show of force in the Persian Gulf since the 2003 invasion of Iraq, led by a pair of aircraft carriers and backed by warplanes off the coast of Iran.
U.S. Navy Cmdr. Kevin Aandahl told AP the 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. (Full story)
Hard-liners in Iran have urged the government to charge the 15 marines and sailors with espionage and put them on trial.
No official word has emerged on whether Iran's government will do so.
But, according to AP, Iran has begun questioning the sailors and marines.
"It should become clear whether their entry was intentional or unintentional. After that is clarified, the necessary decision will be made," AP quoted Iranian Deputy Foreign Minister Mehzi Mostafavi as saying.