Gholam-Reza Ansari told Russian television that Iran had launched a legal investigation of the British sailors.

"They will be tried if there is enough evidence of guilt, " Iran's state news agency IRNA reported.

Meanwhile, the European Union has demanded that Iran immediately and unconditionally release the soldiers and warned of undefined "appropriate measures" if Tehran does not comply.

A statement from EU foreign ministers in Bremen, Germany, yesterday backed British prime minister Tony Blair's statements that the naval personnel were not in Iranian waters when they were seized a week ago after they searched a civilian vessel.

Javier Solana, the EU's foreign policy chief, said he would be in contact with the Iranian leadership "in the coming days, in the coming hours" over the issue.

He said he had already talked to Iran's top nuclear negotiator, Ali Larijani, and foreign minister Manouchehr Mottaki, and would try to contact president Mahmoud Ahmadinejad.

"The leaders of Iran have to understand this is not a bilateral issue" between them and Britain, Solana said. "The government of Iran has to understand that whatever happens to any citizens of the EU. . . happens to the entire union.

This is a mistake, this should not happen, this is not the way in which a country that wants to be a member of the international community should behave, " he said.

The ministers also issued a statement calling on Iran to "immediately inform" the British government of the whereabouts of the captives and give British diplomats access to them, a British request that Iran so far has denied.

The EU statement said that "all evidence clearly indicates that at the time of the seizure, the sailors were on a routine patrolling mission in Iraqi waters" in accordance with a UN mandate. The seizure, the statement said, "therefore constitutes a clear breach of international law. . . The European Union repeats its call for the immediate and unconditional release of the British Royal Naval personnel".

If the British sailors are not released, the 27-member EU "will decide on appropriate measures", the statement said.

Catherine Colonna, French European affairs minister, said "the message to Iran is clear. Today we have given proof of our solidarity with Great Britain". She added the EU ministers will return to the issue in two weeks if it is not settled by then.

Despite the strong words in the statement, some European diplomats warned against unnecessarily blocking normal relations with Iran and escalating the dispute.

"We must put very strong pressure on the Iranians, " French foreign minister Philippe Douste-Blazy, who did not attend the Bremen conference, said in a radio interview in Paris. "I think we must avoid confrontation and escalation."

Luxembourg's foreign minister Jean Asselborn echoed that stand, saying on German radio that "we must watch out. . . that we are not always outbidding each other with sanctions or talking about freezing relations. I believe we are very much in solidarity with Britain in this case.

We must naturally do everything we can to put on the necessary brakes, so that it doesn't explode."