British Prime Minister Tony Blair on Tuesday warned Iran that efforts to secure the release of 15 sailors would enter a "different phase" if diplomatic efforts failed.
Britain has sought to keep up pressure on Iran which has rejected growing international calls for their release.
"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," 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," he said.
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 had 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.
"At the moment, what we are trying to do is to make sure that that diplomatic initiative works," Blair said, adding: "There is absolutely no justification whatever for holding them.
"Our first concern is for their welfare and to get them released as quickly as possible," Blair said, adding that that there were no new details on the sailors' welfare.
Pressed on what a "different phase" 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.
"The most important thing is their welfare, I am trying to get this resolved in as diplomatic and sensible a way as possible."
Blair said he hoped the seizure was not linked to the capture by US forces of five Iranians in Iraq.
"It should have absolutely no bearing at all, because any Iranian forces who are inside Iraq are breaching the UN mandate and undermining the democratically-elected government of Iraq, so they have got no cause to be there at all," he said.
"The two situations are completely distinct.
'Iran considering its options'Citing unnamed sources, the BBC said that 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.
The crisis over the detentions comes as tensions also rise over Iran's disputed nuclear programme.
The European Union has demanded the sailors' release and the United States has expressed its "concern and outrage."
Sir Richard Dalton, the former British ambassador to Tehran, said Blair should prepare to "play a long game and to keep at it."
"I doubt whether there's a viable military solution to this question," he told GMTV.
"We don't know where they are held and we would be going into territory where any UK forces would be heavily outnumbered.
"I think Iran is considering its options."
The family of the sole woman among the eight sailors and seven marines, 26-year-old Faye Turney, said in a statement Monday that they were going through a "very distressing time" and that they were "grateful for the support shown to us."
Earlier, Britain's ambassador held more talks at the foreign ministry in Tehran, while his opposite number in London was summoned to the British foreign ministry for third time for a "frank" discussion.