But Britain's delicate diplomatic efforts were set back by U.S. President George W. Bush, who made a statement Saturday in which he characterized the imprisoned sailors as “hostages” — a phrase that Britain has been carefully avoiding to prevent the crisis from becoming a broader political or military conflict.

Officials in London said that they believe their “confidence-building” operations, in which they offer to guarantee the Iranian government that British vessels will not stray into Iranian waters, offer the best hope of winning the freedom of the sailors and marines who have been in custody since they were seized by Iran's Republican Guards on March 23. Iran says the sailors had strayed into Iranian waters. Britain says they remained in Iraqi waters, where they are allowed to operate in support of the war in Iraq.

“The British hostages issue is a serious issue because the Iranians took these people out of Iraqi waters, and it's inexcusable behaviour,” Mr. Bush said in response to a reporter's question during a press conference at the Camp David retreat.

British negotiators believe the Iranians have already won all the rewards they have been seeking — mainly by using several of the hostages for propaganda purposes by broadcasting videos and letters in which they admit, possibly under duress, to trespassing on Iranian territory and demand that their government withdraw from Iraq.