Joel Johnson, an international-affairs analyst at The Teal Group, says Iran Air "has seven or eight ancient 747s that have been quietly maintained with needed parts by a European carrier over the years, with implicit approval of the U.S."

Joel Johnson, an analyst with Virginia-based Teal Group, said that U.S. officials viewed the sale of spare aircraft parts as a powerful carrot for Iran, which for decades has relied on parts obtained on the black market or copied locally.

He said the move could also help American companies to put themselves in a position to benefit if a broader softening of sanctions is agreed.

"It allows some U.S. companies to get a foot in the door and restore relations that they have not had for over 20 years," Johnson said.