Congress and the Obama administration are stepping up pressure on private companies to stop doing business with Iran, but their efforts are running into the same problems U.S. sanctions have encountered for three decades — reluctance in Europe and a host of elusive trading companies eager to sell gasoline and other goods to Iran.

To deter Iran from developing a nuclear weapons program, the House and the Senate have approved separate bills that would impose sanctions on companies that supply gasoline to the Islamic republic and on the insurance, reinsurance and shipping companies that facilitate such trade. And in the past few weeks, a variety of companies — including Ingersoll Rand, Caterpillar, Siemens and Huntsman — have indicated they will stop or have stopped doing business with Iran.

But a senior European Union official has written to Secretary of State Hillary Rodham Clinton voicing opposition to congressional efforts to impose U.S. sanctions on European companies.

In a Feb. 26 letter, Catherine Margaret Ashton, the E.U.’s high representative for foreign affairs and security policy, expressed “the EU’s deep concern” about the bills because they “envisage the extraterritorial application of U.S. legislation and would therefore be contrary to the EU-U.S. understanding of 1998, under which it was agreed that such sanctions would not be applied to the EU in the light of the EU’s commitment to work with the U.S. to counter the threat that Iran poses to international security.”