EU foreign ministers have demanded the immediate release of 15 British navy personnel seized by Iran a week ago.
The Bremen meeting did not bring specific measures against Iran
The 27 ministers voiced "unconditional support" for Britain in the dispute, in a statement agreed at a meeting in the north German port city of Bremen.
They urged "the immediate and unconditional release" of the crew.
The EU said it reserved the right to take "appropriate measures" if Iran did not comply - though the measures were not spelled out.
The BBC's Oana Lungescu in Bremen says the strongly-worded EU statement goes much further than the UN's expression of "grave concern" about the Iran-UK dispute on Thursday.
Germany hosted the EU meeting as it currently holds the EU presidency.
Earlier, French Foreign Minister Philippe Douste-Blazy described Iran's detention of the Royal Navy crew as "a very serious and unacceptable act which we immediately condemned".
"We stand shoulder-to-shoulder with the British," he added.
The UK Foreign Secretary, Margaret Beckett, did not explicitly ask for a suspension of EU business ties with Tehran.
France and other big European countries, including Germany and Italy, have important economic interests in Iran and would be reluctant to heed such calls, our reporter says.
The European external affairs commissioner, Benita Ferrero-Waldner, warned that the EU had to be careful at a very delicate moment in relations with Iran.
Europe should make clear where it stood, she said, but also hold the door open to negotiations on Tehran's controversial nuclear programme.
The foreign minister of Turkey, whose country is trying to intervene on the UK sailors' behalf, was also invited to the meeting in Bremen.
The ministers also discussed the Middle East and Kosovo.
A paper prepared for the meeting says an EU police mission - the largest of its kind set up by the bloc - could be in place in Kosovo for at least two years and the international community will need to raise $2bn (£1bn) to prop up the province's fragile economy.