Both letters from Turney, a 26-year-old mother and wife from south-west England, were in stilted English, leading some linguistic experts to suggest the text may have been written originally in Farsi and translated into English.

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The spiraling dispute pushed oil prices up more than 3 percent to $66 a barrel on Thursday on worries oil supplies could be affected, and stoked Middle East tensions, already heightened over concerns about Iran's nuclear ambitions.

Hoping to use international channels to force Iran's hand, Britain wanted the UN Security Council on Thursday to adopt a tough stance. But after hours of negotiations, Russia blocked a statement that would have demanded an immediate release of the British crew. This resulted in the more softly worded outcome.

London now plans to raise the matter at the meeting of EU foreign ministers in Bremen, Germany.

Britain, which has frozen all diplomatic business with Tehran apart from discussions over the prisoners, will ask foreign ministers from the 27-nation bloc to follow its lead and adopt tough measures, government sources said.

Iran, however, said its dispute with Britain should be solved through bilateral routes.

"The British government's attempt to engage third parties, including the Security Council, with this case is not helpful," Iran's UN mission said in a statement.

The British Foreign Office spokeswoman said Britain's embassy in Tehran received the formal note from the Iranian government, but she was unable to say when.

"Such exchanges are always confidential, so we cannot divulge any details," she said.

In a further development on Thursday, Iranian state television said President Mahmoud Ahmadinejad would consider a Turkish request to free Turney.

It said Turkish Prime Minister Tayyip Erdogan asked Ahmadinejad to free her in a telephone call.

In a similar incident, Iran freed eight British service members after holding them for three days in 2004, a time of less tension with the West. In 1979, Iranian students seized the US embassy in Tehran and captured 90 hostages; 52 were held captive for 444 days.

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