The government is attempting to “discreetly” talk to the Iranians to secure the release of 15 Royal Navy personnel, Downing Street has said.
Tony Blair’s spokesman said that if the talks were unsuccessful, the government may have to become “more explicit”.
He said they were “utterly confident” the 15 had been in Iraqi, not Iranian, waters, when they were captured.
Meanwhile, Foreign Secretary Margaret Beckett will shorten a visit to Turkey to fly home to help manage the crisis.
The 15 sailors and marines from HMS Cornwall were captured on Friday after searching a boat in the Gulf, off the coast of Iraq, which they suspected was smuggling cars.
Iran says the British personnel were trespassing in Iranian waters when they were seized – but the prime minister said the group were in Iraqi waters under a UN mandate.
The prime minister’s spokesman said the matter was being dealt with “privately” but the Iranians could be “in no doubt that we expect the immediate release of our personnel”.
Earlier, Mr Blair warned of a “different phase” if diplomacy failed to secure their release.
His spokesman said he was referring to a “different way” of handling talks, which could involve making public reasons why the UK was certain the group was in Iraqi waters.
It is understood this could include producing evidence such as boat co-ordinates and details of the searched vessel apparently still anchored in Iraqi waters.
The spokesman told reporters: “We are utterly confident that we were in Iraqi waters, and not just marginally in Iraqi waters but in Iraqi waters. It’s a case of tactics and if and when we have to prove that.”
However, one high-ranking Iraqi official has expressed surprise that British forces were operating in the area.
Brigadier-General Hakim Jassim, commander of Iraq’s territorial waters, said: “Usually there is no presence of British forces in that area, so we were surprised and we wondered whether the British forces were inside Iraqi waters or inside Iranian regional waters.”
The BBC has been told the group are being held at an Iranian Revolutionary Guards Corps base in Tehran.
They have been held for five days, but are said to be being treated humanely.
On Tuesday, Defence Secretary Des Browne chaired a meeting of ministers and officials – under the auspices of the government’s “civil contingencies committee” known as Cobra – to discuss the situation.
Officials said it was intended to ensure coordination across Whitehall and keep civil servants updated on the latest developments.
Cobra leads responses to national crises and convened in recent years for the 7 July London bombings, the fuel protests and 11 September attack.
It is understood that while still in Turkey, Mrs Beckett spoke to Iran’s Foreign Minister Manouchehr Mottaki to push again for immediate consular access to them.
The Associated Press news agency reported that Iranian officials had repeated assurances that British diplomats would get access to the detainees once their inquiry into the incident was complete.
On Wednesday, Mrs Beckett is expected to make a statement to the Commons.
Faye Turney, one of the 15 captured, was interviewed by the BBC last week.
She said: “Sometimes you may be called upon, and when you do you’ve just got to deal with it and get on with it”.
Meanwhile, her family, from Shrewsbury, Shropshire, have said it is a “very distressing time” for them.