LONDON — European firms, fearing a loss of their U.S. market, have shied away from signing major contracts with Iran.

European industry sources said British, French and German majors have refused the appeals by their governments to negotiate major contracts with Iran as part of an effort to woo Teheran away from the construction of nuclear weapons. The sources said these companies fear loss of their U.S. market upon signing any agreement with Teheran.

"The EU has been too late," an industry source said. "The Americans have already warned European majors to keep out of Iran, or else."

U.S. officials have confirmed that the Bush administration warned leading European companies of sanctions if they sign major deals with Iran. The officials said the administration wants to ensure a permanent halt in Iranian uranium enrichment and full International Atomic Energy Agency access to suspected nuclear facilities before easing a ban on trade with Teheran.

"We want any proliferators, from multinational conglomerates to small exporters of dual-use machine tools, to understand that the U.S. will impose economic burdens on them, and brand them as proliferators," U.S. envoy to the IAEA Jackie Sanders said in November 2004.

The industry sources said the United States has stressed that it would not tolerate any European help to Iran's nuclear program. Britain, France and Germany have pledged to facilitate the construction of a light-water nuclear reactor if Teheran ends its uranium enrichment program.

So far, leading firms in France and Germany have told their governments they would not participate in such an Iranian program. The sources said the firms included Siemens, the original contractor of Iran's nuclear reactor at Bushehr. Siemens was replaced by Russia in the 1990s.