The Trump administration on Monday announced sanctions on Mojtaba Khamenei, the second son of Iran’s Supreme Leader Ali Khamenei, and eight other advisers, among them Ali Akbar Velayati and the head of judiciary Ebrahim Raisi.
Washington has also offered a reward of $20 million for information to secure the release and return of Robert Levinson, the longest-held US hostage, last seen in Iran in 2007.
The announcement came on the 40th anniversary of the Iranian hostage crisis, when 52 American diplomats and citizens were held hostage at the US embassy in Tehran for 444 days from November 4, 1979.
The sanctions announced by the Treasury Department on Monday target “Khamenei’s network”, specifically appointees in the Office of the Supreme Leader, the Expediency Council, the Armed Forces General Staff, and the Judiciary.
US Secretary of State Mike Pompeo said: "The designation seeks to block funds from flowing to a shadow network of Khamenei’s military and foreign affairs advisers who have for decades oppressed the Iranian people, supported terrorism, and advanced destabilising policies around the world.
"While the Iranian regime’s decision to jail our diplomats has cast a 40-year shadow over our relations, the United States knows that the longest-suffering victims of the Iranian regime are the Iranian people.
"We wish nothing more for them than a future with a truly representative government and friendship with the American people."
He said two those designated have been linked to the 1983 US Marine barrack bombing in Beirut that killed 241 personnel and the 1994 bombing of Argentine Israelite Mutual Association.
Those targeted besides Mr Raisi and Mojtaba Khamenei and Mr Velayati are: the Supreme Leader’s Chief of Staff Mohammadi Golpayegani; close associate Vahid Haghanian; Gholam Ali Hadad Adel the father-in-law of Mojtaba Khamenei, and a member of the Expediency Council; Iranian Revolutionary Guard Brigadier General Hossein Dehghan; and Gholam Ali Rashid, an IRGC commander.
The US also sanctioned Iran’s Armed Forces General Staff and its chief, General Mohammad Bagheri.
The US also issued a $20 million reward for information leading to return of hostage Mr Levinson:
In a statement, Mr Levinson's family said the $20 million reward, combined with the $5 million reward already in place, was one of the highest in history.
They said: "This sends a clear message from our government of how important it is that Bob Levinson be returned to his family and friends who love him. All the Iranian authorities need to do is send him home.
"To the individuals in Iran and elsewhere who know where Bob Levinson is, or have information that will bring him back to us, please contact the nearest U.S. embassy or consulate. All we care about is having him home, so he can live the rest of his life in peace."
"Bob Levinson must come home, and Iran's hostage-taking as government policy must end.
"We wish to express our deep appreciation to President Trump and everyone in the Administration who is working hard every day to bring him home."
A US senior official said the steps coincide with the 40th anniversary of the hostage crisis in 1979.
“The regime revealed its true colours 40 years ago,” the US official said. The sanctions are a “grim reminder that it [regime] is still the same," he added.
The official did not comment on possible negotiations with Iran on at least five US hostages now being held, but said “they should immediately on humanitarian grounds release hostages”.
Mr Pompeo added: "Forty years ago today, heavily armed followers of Ayatollah Khomeini broke the gates and scaled the walls of the US Embassy in Tehran.
"More than 50 US diplomats were taken hostage by the Iranian regime for 444 days. For almost 15 months, these courageous and resilient Americans and their families did not know if they would live or die. Thankfully, they lived.
"Forty years later, the revolutionary regime in Tehran has proven, time and again, that its first acts after gaining power were a clear indication of its evil character. The regime continues to unjustly detain Americans and to support terrorist proxy groups that engage in hostage taking.
"The Trump administration has made clear that the regime in Iran must release all missing and wrongfully detained Americans, including Robert Levinson, Xiyue Wang, Siamak Namazi, and others. We will not rest until they are reunited with their families."