British Prime Minister Tony Blair has warned Iran that efforts to free 15 British naval personnel will enter a "different phase" if diplomacy fails.
Mr. Blair did not provide details when asked to elaborate during an interview with GMTV. But he did say that he hopes Iran can be persuaded through diplomatic channels that there is no justification for holding the 15 Britons.
Iran captured the 14 men and one woman Friday and is interrogating them. Iran says they entered Iranian waters, but Britain and Iraq insist they were operating in Iraqi waters when they were seized.
British Foreign Secretary Margaret Beckett cut short a visit to Turkey Tuesday so she could report to parliament on the crisis. Beckett said she was returning to London, because the situation with Iran has not been resolved yet as she had hoped.
The White House says it shares Britain's "concern and outrage" over the seizure of the British naval personnel.
Iranian forces detained the crew after the British conducted a routine inspection of a merchant ship in the Shatt al-Arab waterway between Iran and Iraq.
Three years ago, Iranian forces seized six British marines and two sailors in the same area. They were released three days later.
Some information for this report was provided by AP and Reuters.