March 27 (Bloomberg) -- U.K. Prime Minister Tony Blair warned of a ``different phase'' of efforts to release 15 British sailors and Marines seized by Iran if negotiations fail.

The U.K. is using diplomatic channels to make the Iranian government understand that the eight Royal Navy sailors and seven Royal Marines ``have to be released,'' Blair said today in an interview with the U.K. television show GMTV.

``We cannot have a situation where our servicemen and women are seized,'' Blair said. Asked what he meant by a different phase, Blair didn't elaborate. ``We'll just have to see,'' he said.

Iran's Foreign Ministry spokesman, Mohammad Ali Hosseini, said Britain will be allowed to make contact with the sailors and marines as soon as a preliminary investigation has been completed, Cable News Network reported.

The group was captured at gunpoint in the Shatt al-Arab waterway, which divides Iran and Iraq, on March 23. The U.K. says they were in Iraqi waters conducting routine inspections of merchant shipping. Iran says they were in Iranian waters.

The incident has led to a heightening of diplomatic tensions between the U.K. and Iran, which is already under international pressure over its refusal to suspend its nuclear program. The country has also been accused by Blair and President George W. Bush of interfering in Iraq.

Iranian Diplomats

On Jan. 11, U.S. forces arrested five Iranian diplomats in the northern Iraqi city of Arbil. They were connected to the Iranian Revolutionary Guards Corps' Quds faction that reports to Iran's supreme leader, Ayatollah Ali Khamenei, the U.S. said.

Asked if Iran's decision to detain the British personnel was a direct result of that incident, Blair said ``it should have absolutely no bearing at all'' and the two situations were ``completely distinct.''

His spokesman Tom Kelly later told reporters that when the Prime Minister referred to a ``different phase'' in dealing with Iran, he didn't mean military action or expelling diplomats. Blair would prefer the matter to be resolved ``quietly and privately'' and the new phase may involve making public why the U.K. was so sure its people were in Iraqi waters, Kelly said.

``We have been clearly stating that we are utterly certain that the personnel were in Iraqi waters,'' he said. ``We so far haven't made explicit why we know that because we don't want to escalate this. We may get to the stage where we will become more explicit.''

The standoff may escalate tensions between Iran and coalition forces, said Dan Plesch, director of the Center for International Studies at the School of Oriental and African Studies in London.

U.S. Aircraft Carriers

``The Iranians are grossly overplaying their hand, assuming the U.S. doesn't have the political will or the military might to destroy Iran, and they have both,'' Plesch said in an interview. ``The only consequence I can see of putting more and more pressure on Iran is we get closer and closer to full-scale military action.''

The U.S. Navy has begun an exercise, involving two aircraft carriers and more than 100 planes, which is its largest show of force in the Persian Gulf since the 2003 invasion of Iraq, the Associated Press reported.

It wasn't organized in response to the capture of the British forces, and the warships will remain outside Iranian waters, which extend 12 miles (19 kilometers) from Iran's coast, AP said, citing an unidentified U.S. Navy commander.

U.K. Foreign Secretary Margaret Beckett, speaking at a news conference in the Turkish capital, Ankara, called for the ``safe and speedy return'' of the Britons and for U.K. diplomats to be given access to the group.

Statement to Parliament

Beckett will return to the U.K. earlier than planned in order to make a statement to Parliament on the crisis tomorrow, the Foreign Office in London said in a statement. She spoke in ``very robust terms'' to the Iranian foreign minister by telephone today, the Foreign Office said.

``The issue will be solved in a calm atmosphere,'' said Hosseini, Iran's Foreign Ministry spokesman, in a phone interview today. ``We cannot predict how long it will take to resolve the case'' said Hosseini, speaking from Tehran.

The Britons are being held at an Iranian Revolutionary Guards Corps base in Tehran, the British Broadcasting Corp. reported, without saying where it got the information.

One of the seized Britons was identified today as Faye Turney, a 26-year-old mother of a young daughter.

``This is a very distressing time for us and our family,'' her relatives said in a joint statement released through the U.K.'s Ministry of Defence. ``We are grateful for the support shown to us by all personnel involved and appreciate it.''

Turney was interviewed by the British Broadcasting Corp. only last week. ``Sometimes you may be called upon and when you do you've just got to get on with it,'' she said.

To contact the reporters on this story: Nick Allen in London at nallen14@bloomberg.netRobert Hutton in London at rhutton1@bloomberg.net

Last Updated: March 27, 2007 16:43 EDT