WASHINGTON - The United States for the first time Tuesday implemented sanctions to strangle Iran's importation of gasoline in a major tightening of measures that is aimed at cutting off money for Tehran's nuclear program but that could hurt ordinary Iranians.

The sanctions were imposed on Venezuela's state-owned oil company, an Israeli shipping firm, and five other foreign companies that the State Department said had been doing business with Iran's energy sector.

"All of these companies have engaged in activities related to the supply of refined petroleum products to Iran, including the direct supply of gasoline and related products," Deputy Secretary of State James Steinberg said.

It is the first time the United States has imposed sanctions designed to curtail Iran's imports of gasoline and other refined petroleum products. Although Iran is one of the world's major oil producers, it lacks refinery capacity and must import an estimated 40 percent of its gasoline.

U.S. officials contend that Iran's petroleum sector is a key source of funding for the country's ballistic-missile program and its uranium-enrichment program, which Western governments charge is a cover for a secret nuclear-weapons development effort.

Iran vehemently denies the allegation, asserting that it is enriching uranium for use in nuclear power plants.

The sanctions are aimed at intensifying pressure on Tehran to comply with U.N. resolutions demanding that it stop enriching uranium and reengage in long-stalled negotiations with the United States, China, Russia, Britain, France, and Germany.

The United States previously had refrained from going after Iran's gasoline imports under a sanctions strategy that targeted Iranian officials, government organizations, and companies but that avoided steps that could intensify the severe economic hardships that afflict ordinary Iranians.

President Obama, however, has been under pressure from Congress to implement the Comprehensive Iran Sanctions, Accountability and Divestment Act of 2010, which he signed nearly a year ago and which authorizes measures to restrict Iran's gasoline imports.

He signed an executive order Monday to implement the act.