“If you don’t put yourself in a position where you can act on the pressure track, you’ll be less likely to have the engagement track work,” said a high-ranking administration official, who spoke on the condition of anonymity because of the delicacy of the matter. “The key is for the Iranians to see both sides of this.”

To that end, he said, the United States is refusing to take any measures off the table, even an embargo of gasoline and other refined fuel. European countries fear such measures would inflict misery on the Iranian people by creating shortages that would make them angry at the West and drive them into the arms of an unpopular government.

“We’re not going to be in the business of pre-emptively excluding anything,” the American official said. “The key is not what ought to have an impact on them, but what in their eyes will have the most impact on them.”

For several months, officials said, the administration has been talking with other countries about a list of potential sanctions against Iran. Congress is weighing legislation to expand financial sanctions, while the chairman of the House Foreign Affairs Committee, Representative Howard L. Berman, Democrat of California, is pushing a bill that would levy sanctions on foreign companies that export gasoline to Iran.

Existing sanctions have had some practical consequences. In Tehran, the power goes out regularly, a result of a shortage of electricity generators, which are on the list of prohibited goods because of their potential for military use. That did not happen even a decade ago.

Still, the experience of the United States in policing its own unilateral economic sanctions  which were imposed after the 1979 hostage crisis and expanded several times since then  shows that restrictions are more likely to drive up prices of banned foreign goods than to stop them totally from flowing into Iran. The world is full of rogue nations and smugglers skilled at hiding illicit goods in the vast stream of global trade, and other nations may not be as willing as the United States to police sanctions rigorously.

The guilty plea by the Dutch avionics company late last month was just the latest in a string of indictments against several hundred such defendants since 2007, when the Bush administration significantly increased efforts to enforce the United States’ own trade restrictions with nations like Iran.