In one instance, an American company was permitted to bid on a pipeline job that would have helped Iran sell natural gas to Europe, even though the United States opposes such projects. Several other American businesses were permitted to deal with foreign companies believed to be involved in terrorism or weapons proliferation. In one such case, involving equipment bought by a medical waste disposal plant in Hawaii, the government was preparing to deny the license until an influential politician intervened.

In an interview, the Obama administration’s point man on sanctions, Stuart A. Levey, said that focusing on the exceptions “misses the forest for the trees.” Indeed, the exceptions represent only a small counterweight to the overall force of America’s trade sanctions, which are among the toughest in the world. Now they are particularly focused on Iran, where on top of a broad embargo that prohibits most trade, the United States and its allies this year adopted a new round of sanctions that have effectively shut Iran off from much of the international financial system.

“No one can doubt that we are serious about this,” Mr. Levey said.

But as the administration tries to press Iran even harder to abandon its nuclear program — officials this week announced several new sanctions measures — some diplomats and foreign affairs experts worry that by allowing the sale of even small-ticket items with no military application, the United States muddies its moral and diplomatic authority.

“It’s not a bad thing to grant exceptions if it represents a conscious policy decision to give countries an incentive,” said Stuart Eizenstat, who oversaw sanctions policy for the Clinton administration when the humanitarian-aid law was passed. “But when you create loopholes like this that you can drive a Mack truck through, you are giving countries something for nothing, and they just laugh in their teeth. I think there have been abuses.”

What’s more, in countries like Iran where elements of the government have assumed control over large portions of the economy, it is increasingly difficult to separate exceptions that help the people from those that enrich the state. Indeed, records show that the United States has approved the sale of luxury food items to chain stores owned by blacklisted banks, despite requirements that potential purchasers be scrutinized for just such connections.