LONDON - Britain will seek on Friday to step up international pressure on Iran to release 15 British sailors and marines seized a week ago, and said it would reply soon to an Iranian government message about the crisis.

Faye Turney, the only woman crew member of the 15 British sailors and marines detained at sea last week, speaks during an interview on Iranian television March 28, 2007. [Reuters]

After a day of escalating tension between London and Tehran, which pushed oil prices up sharply, the UN Security Council agreed a watered down statement expressing "grave concern" at the situation and supporting calls for the crew's release.

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Britain, which had sought a tougher statement, plans to urge the European Union to help isolate Iran at a two-day meeting of EU ministers starting on Friday.

A British Foreign Office spokeswoman said Britain was also preparing a response to a note from the Iranian government.

"...We are giving the message serious consideration, and will soon respond formally to the Iranian government," she said, without giving details.

Iran's IRNA news agency said the Iranian Foreign Ministry had sent a message to the British embassy in Tehran asking for "necessary guarantees that violations against Iranian waters would not be repeated."

Britain and Iran are at odds over whether the 15 Britons were in Iranian or Iraqi waters when captured carrying out patrols authorized by the United Nations and Iraq's government.

London says satellite data prove last Friday's seizure happened in Iraqi waters, but Tehran has video footage and charts it says show the event took place in Iranian waters.

Ratcheting up emotions, Iran on Thursday released a second letter purportedly by the only female member of the crew, Faye Turney, in which she again said her patrol had "entered into Iranian waters."

Britain reacted angrily to what it said was blatant propaganda, labeling the note's release "outrageous and cruel."

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