The New York Times reported that the Bush administration has cautioned energy companies and foreign governments that they may face sanctions if they make energy deals with Iran.
That could leave some big Iranian energy projects up in the air, the Times reported.
One is a $10 billion natural gas project planned by Shell and the Spanish oil company Repsol, the newspaper said. U.S. Ambassador to Spain Eduardo Aguirre Jr. met with Repsol executives in Madrid last month and advised them against moving ahead on the project, the Times reported.
"No investment is being made at present," a Repsol spokesman told The Times.
U.S. officials suspect Iran's nuclear program is preparing to build weapons, although Iran says it is only to generate power.
The warnings may not be enough to satisfy members of Congress who want to see mandatory sanctions.
"This administration has done nothing to punish Iran," Rep. Tom Lantos, D-Calif., told the Times. "The method I don't favor on Iran is to bomb their nuclear facilities. The method I favor is to starve them of resources, which can only be done through sanctions."