European Union nations have formally adopted an oil embargo against Iran as part of sanctions over its nuclear programme.

British Foreign Secretary William Hague this morning called the measure part of “an unprecedented set of sanctions.”

He says, “I think this shows the resolve of the European Union on this issue.”

Diplomats say the measures, adopted by the EU’s 27 foreign ministers, include an immediate embargo on new contracts for crude oil and petroleum products while existing ones are allowed to run until July.

Iran says its nuclear program is exclusively for peaceful purposes. But many international officials fear the country is trying to develop nuclear weapons.

The EU will also likely freeze the assets of the Iranian central bank.

“The pressure of sanctions is designed to try and make sure that Iran takes seriously our request to come to the table,” EU foreign policy chief Catherine Ashton said.

Negotiators have worked hard to try to ensure that the embargo punishes only Iran — and not EU member Greece, which is in dire financial trouble and relies heavily on low-priced Iranian oil.