BRUSSELS — The European Union intensified the economic isolation of Iran over its nuclear program by ordering its toughest economic sanctions yet against the Iranian government on Monday. European energy companies and insurers affected by the new controls promised to comply.

Although European companies will still be allowed to import oil and gas from Iran, the sanctions go beyond those in the fourth round imposed by the United Nations last month. The European move follows the imposition of additional sanctions by the United States.

Meant as punishment for Iran's refusal to halt its enrichment of uranium, the measures appeared to prompt a flurry of diplomatic activity. Iran said it was ready to return to talks on a nuclear fuel swap, but it was unclear whether the offer was anything more than another in a series of maneuvers by Tehran to buy more time.

Western nations suspect that Iran is trying to build a nuclear weapon; the country insists that its nuclear program is peaceful and that sanctions will not persuade it to change course.

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While American investment in Iran has dwindled, the European Union, with 27 member nations, is Iran's largest trading partner, taking in a third of its exports and selling it billions of dollars in goods and services.

That means that the measures could have a significant impact, though it was too early to determine the financial effect on European companies and the Iranian economy. The European Union's foreign policy chief, Catherine Ashton, said figures on the amount of trade that would be involved in the sanctions were still being assessed.

The 27 European foreign ministers, in a statement, said they had agreed on "a comprehensive and robust package of measures in the areas of trade, financial services, energy, transport, as well as additional designations for visa ban and asset freeze." They said the penalties would focus on Iranian banks, the Islamic Revolutionary Guards Corps and the Islamic Republic of Iran Shipping Lines.

Details of the companies and individuals covered by European measures have not yet been released.

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