One factor behind the warnings, administration officials acknowledge, is that the Democratic-controlled Congress appears to be moving quickly to pass a law that would make sanctions mandatory out of concern about Iran’s suspected nuclear arms programme and support for terrorism. “What we’re trying to do is create multiple points of pressure on Iran in both the private and public sector,” said R Nicholas Burns, Undersecretary of State for Political Affairs. “These companies also need to know that the attitude of Congress on their activities in Iran is hardening.”

Last month, the US ambassador to Spain, Eduardo Aguirre Jr., met with Repsol executives in Madrid to advise them against going forward with a deal to develop Iran’s South Pars field, which contains one of the world’s biggest natural gas deposits. The ambassador was told that the deal was not yet final, according to American and Repsol officials.

The messages to oil companies mark the latest episode in a long campaign of pressure that reached a turning point in December, when the administration won approval of a UN Security Council resolution designating 10 Iranian companies and a dozen individuals as off-limits for international financial dealings.

Another resolution designating an additional 15 individuals and 13 Iranian government and business groups, including a leading Iranian bank, could be approved later this week. The administration, using the Security Council list, wants virtually all of the world’s banks and businesses to boycott all of these Iranian entities.

Despite the stepped up American pressure, some Democratic leaders in Congress are complaining that the administration has not gone far enough. They want Bush to invoke a statute enacted in 1996 that obliges the US government to punish any foreign energy company doing business with Iran unless the president waives the sanction on national security grounds. “This administration has done nothing to punish Iran,” said Rep. Tom Lantos, D-Calif., who is chairman of the House Foreign Affairs Committee. “The method I don’t favour on Iran is to bomb their nuclear facilities. The method I favour is to starve them of resources, which can only be done through sanctions.”

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