( AFP ) - British Prime Minister Tony Blair warned Iran Tuesday that efforts to secure the release of 15 sailors would enter a "different phase" if diplomatic efforts failed.
His official spokesman said London was not looking to escalate the stand-off and would prefer to resolve the spat quietly, insisting that Britain was not considering military action or throwing out Iran's ambassador.
But London is clearly seeking to keep up the pressure on Tehran, which has rejected growing international calls to freee the naval personnel.
Britain was trying to "pursue this through the diplomatic channels and make the Iranian government understand these people have to be released," Blair told GMTV television.
"I hope we manage to get them to realise they have to release them. If not, then this will move into a different phase."
Pressed on what that might involve, 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 United Nations mandate, patrolling perfectly rightly and in accordance with that mandate, and then effectively captured and taken to Iran."
Blair said that the sailors' welfare was paramount.
"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," he said.
"There is absolutely no justification whatever for holding them."
Britain, supported by Iraq, insists that the naval personnel were conducting "routine" anti-smuggling operations in Iraqi waters Friday when they were seized at gunpoint in the Shatt al-Arab waterway in the north of the Gulf.
Iran says they entered its territorial waters illegally and the BBC, citing sources, said they were now being held in Tehran, where they were being interrogated by the Revolutionary Guards.
Blair's spokesman said London was "utterly certain" that the 15 navy personnel were in Iraqi waters Friday when they were detained in the Shatt al-Arab waterway between Iran and Iraq.
"So far, we haven't made explicit why we know that because we don't want to escalate this," he said, adding: "We don't want to do that too soon because we prefer to have this ... resolved quietly."
But he added: "We may come to the stage where we have to become more explicit about why we know this."
Speaking in Ankara, Foreign Secretary Margaret Beckett reiterated that there was no doubt the sailors had been in Iraqi waters whne they were seized.
"There can be of course no charge of espionage for someone operating in a country's waters in support of the government of that country," Beckett said.
Citing unnamed sources, the BBC said the crew were being grilled at a Revolutionary Guards base in Tehran to find out if they were on an intelligence-gathering mission.
The investigation involved examining tracking equipment to determine exactly where the sailors were seized.
Sir Richard Dalton, the former British ambassador to Tehran, said Blair should prepare to "play a long game and to keep at it."
The European Union has demanded the sailors' release and the United States has expressed its "concern and outrage."
The crisis over the detentions comes as concerns also rise over Iran's disputed nuclear programme.
World oil prices dipped Tuesday after a surge towards 65 dollars a barrel in London amid political tensions over the Islamic republic.
The price of Brent North Sea crude for May delivery had reached 64.64 dollars a barrel, the highest level since December 4 last year.
Industry experts say Iran could disrupt its oil exports should tensions surrounding the Tehran's nuclear programme escalate and the capture of British sailors has upset the market.