"These designations are designed to highlight the fact that on both proliferation grounds and terrorism grounds, the Iranian government is not in compliance with its international obligations," one official said.
Condoleeza Rice, secretary of state, and Henry Paulson, the treasury secretary, announced the measure in full later on Thursday.
Speaking to a congressional committee a day before, Rice said: "The policies of Iran constitute perhaps the single greatest challenge for American security interest in the Middle East and possibly around the world because the combination of Iranian terrorism, Iranian repression at home and the pursuit of nuclear weapons technology, technologies that could lead to a nuclear weapons, is a very dangerous mix."
The United States has long labeled Iran a "state supporter of terrorism", and has been working for years to gain support for tougher sanctions from the international community.
The new sanctions come without international backing and are believed to be the first of their type taken by the United States specifically against the armed forces of another government.
The Washington Post cited senior administration officials as saying the sanctions would be the broadest set of punitive measures against Iran since 1979 and would empower the US 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 US 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.