Secretary of State Condoleezza Rice and Treasury Secretary Henry Paulson were to announce the new steps, aimed at quelling Iran's nuclear ambitions, at a news conference at the State Department later Thursday morning, said the official, who declined to be identified publicly.
Rice told a House committee Wednesday that the administration shares Congress' goal of making sanctions tougher on Iran. She also declared that activities in Iraq by the Quds Force "are inconsistent with the Iranian government's obligations and stated commitment to support the Iraqi government."
The U.S. official confirmed the looming sanctions only on grounds of anonymity because the official announcement was still pending.
Rice and Paulson were ready to outline sanctions that would be the toughest the United States has levied against Tehran since the 1979 takeover of the U.S. Embassy there, said The Washington Post, which first reported the story in Thursday's editions.
The New York Times quoted a senior government official as saying the United States "will be freezing assets and there will be ripple effects of where we can go from there. This is going to be a broad and wide-ranging effort."
All the officials spoke on condition of anonymity in advance of formal announcement of the new sanctions.
The Post said the Revolutionary Guard will be designated as a proliferator of weapons of mass destruction and the Quds Force as a supporter of terrorism, the Post reported.
The United States has long labeled Iran as a state supporter of terrorism and has been working for years to gain support for tougher sanctions from the international community aimed at keeping the country from developing nuclear weapons.
The sanctions being announced Thursday would be unilateral, however, and are believed to be the first of their type taken by the United States specifically against the armed forces of another government.
The sanctions reportedly will empower the United States to financially isolate a large part of Iran's military and anyone inside or outside Iran who does business with it.
Such steps could impact any number of foreign companies by pressuring them to stop doing business with the Revolutionary Guards or risk U.S. sanctions.
The Revolutionary Guards, formed to safeguard Iran's 1979 Islamic revolution, has pushed well beyond its military roots, and now owns car factories and construction firms and operates newspaper groups and oil fields.
Current and former members now hold a growing role across the country's government and economy, sometimes openly and other times in shadow.
The guards have gained a particularly big role in the country's oil and gas industry in recent years, as the national oil company has signed several contracts with a guards-operated construction company. Some have been announced publicly, including a $2 billion deal in 2006 to develop part of the important Pars gas field.
Now numbering about 125,000 members, they report directly to the supreme leader and officially handle internal security. The small Quds Force wing is thought to operate overseas, having helped to create the militant Hezbollah group in 1982 in Lebanon and to arm Bosnian Muslims during the Balkan wars.
The administration accuses the Quds Force of sending fighters and deadly roadside bombs, mortars and rockets to kill American troops in Iraq in recent years - allegations that Iran denies.
The United States pressures U.S. and European banks to do no business with Iranian banks, such as Bank Sedarat that the Bush administration believes help finance guards' business operations. But the United States has been known for some time to also be considering naming the entire group as a foreign terrorist organization, allowing wider financial crackdowns.