WASHINGTON — The Obama administration will hit the Iranian economy with new sanctions today, a U.S. official said, teaming with Britain and Canada in an effort to pressure Tehran to halt its suspected nuclear weapons program.

The financial and energy sanctions will target Iranian companies, the hardline Revolutionary Guard force and Iran’s petrochemicals sector, the official said. The aim would be to build on several American measures already in place to isolate Iran’s economy. The official spoke on condition of anonymity because of the sensitivity of the issue.

The action represents the first direct response to the U.N. nuclear agency’s recent report suggesting Iranian work toward the development of atomic weapons. The report’s release has sparking frenzied international diplomacy over how to halt the Iranian threat, with President Barack Obama pressing the leaders of Russia and China little more than a week ago to join the United States and its allies in taking action.

Obama, however, returned home from an Asia-Pacific trip without any firm commitments from either of the two veto-wielding U.N. Security Council members over stiffer penalties against Iran. The U.S. has insisted since the report’s publication two weeks ago that it was prepared to act unilaterally, or in concert with like-minded governments, to increase the heat on Tehran amid hardening suspicion over its nuclear ambitions.

The U.S. sanctions will likely be announced later today by Secretary of State Hillary Rodham Clinton, the official said. Britain and Canada are also expected to announce new measures against Iran.

The International Atomic Energy Agency’s report alleges Iran has been seeking to acquire equipment and weapons design information, testing high explosives and detonators and developing computer models of a warhead’s core. It is the strongest evidence yet that the Iranian program ranges far beyond enriching uranium for use in energy and medical research, as Iran’s government insists.

The U.S. has already slapped sanctions on dozens of Iranian government agencies, officials, and financial and shipping companies over the nuclear program. The United Nations has passed four rounds of U.S.-backed sanctions against Iran’s economy, but China and Russia appear to stand in the way of any further measures.

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