Tony Blair warned the Iranian government this afternoon about the seriousness of seizing British sailors in Iraqi waters.

The Prime Minister, making his first comments on the capture, said it was “simply not true” that they had been in Iranian territory at the time.

The 15 British sailors and Royal Marines, a woman among them, were arrested by Iran's Revolutinary Guards off the coast of Iraq.

Mr Blair said he hoped the issue could be resolved diplomatically in the next few days, adding: “But the quicker it is resolved, the easier it will be for all of us.”

Speaking in Berlin after attending the EU’s 50th anniversary celebrations, the Prime Minister said: “I have not been commenting up to now because I want to get it resolved in as easy and diplomatic a way as possible, because it is the welfare of the people that have been taken by the Iranian government that is most important. But this is a very serious situation.”

Mr Blair went on: “There is no doubt at all that these people were taken from a boat in Iraqi waters. It is simply not true that they went into Iranian territorial waters, and I hope the Iranian government understands how fundamental an issue this is for us. We have certainly sent the message back to them very clearly indeed. They should not be under any doubt at all about how seriously we regard this act, which is unjustified and wrong.”

Britain does not know where 15 captured Britons are being held, the Government said today as crisis talks continued to secure their release.

Lord Triesman, a Foreign Office minister, speaking this morning as the British ambassador in Tehran held talks with the Iranian regime, repeated demands for the group to be freed.

“We do not know where they are, I wish we did,” Lord Triesman told Sky News amid reports that the sailors and Marines had been taken to Tehran. “We are asking to know whether they are being moved around in Iran.”

The minister, who held an hour of “frank” discussions with the Iranian ambassador yesterday, said: “We have been insisting that they should be released immediately - there is no reason to hold them - that they should be released unharmed and that we should be in a position to assure their families that they are in good health and that they are safe.”

The Foreign Office said this morning’s latest talks in Tehran were held at the request of the British ambassador to Iran, Geoffrey Adams.

Comparing the situation to that of UK embassy staff taken hostage in Ethiopia recently, Lord Triesman said: “These things are always very difficult. They are delicate discussions. My belief is that they will come to a good outcome but you can never be certain.”

Lord Triesman said he had been assured the group was not being held hostage for political reasons as the UK joined other United Nations Security Council members in imposing tougher sanctions on Iran over its refusal to suspend its nuclear programme. “Our own view is that these things are not linked,” Lord Triesman said. He believed it was a “technical” dispute over whether - as Iran claims - the personnel had strayed into Iranian waters or - as the UK insists - were on the Iraqi side of the border.

The troops, from the Type 22 frigate HMS Cornwall, were carrying out a routine search of a large cargo dhow which they suspected of smuggling. A Tehran-based news agency, Fars, claimed the seamen have admitted illegally sailing into Iranian waters and that satellite tracking systems on the British boats proved they were inside Iranian territory.

Lord Triesman said the Naval commander in the area was confident they had the technical evidence to disprove the Iranian claims.

The United Nations Security Council voted unanimously last night to impose tougher sanctions against Iran, hoping to send a strong message to Tehran that it would grow more isolated by refusing to suspend enrichment. The moderately more stringent UN sanctions include banning Iranian arms exports and freezing the assets of 28 additional people and organisations involved in Iran’s alleged nuclear missile programmes. The council voted 15-0 in favour of the proposals.

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