The US Supreme Court has ruled that almost $US2 billion in frozen Iranian assets must be turned over to American families of people killed in the 1983 bombing of a US Marine Corps barracks in Beirut and other attacks blamed on Iran.
The court's 6-2 ruling dealt a setback to Iran's central bank, finding that the US Congress did not usurp the authority of American courts by passing a 2012 law stating that the frozen funds should go towards satisfying a $US2.65 billion ($A3.40 billion) judgment won by the families against Iran in US federal court in 2007.
Bank Markazi had challenged a 2014 ruling by the New York-based 2nd US Circuit Court of Appeals that the assets, bonds held in a trust account overseen by former federal judge Stanley Sporkin, should be handed over to the more than 1000 American plaintiffs.
With the legal questions resolved, lawyers for the plaintiffs said all that is left is for a federal judge to allow Sporkin to distribute the funds.
The lead plaintiff was Deborah Peterson, whose brother, Marine Lance Corporal James Knipple, died in the Beirut bombing.
Peterson said for her the legal fight has never been about just the money.
"The mission was for those responsible for the bombing to be held accountable and for the world to understand what happened in Beirut," Peterson said.
Ted Olson, the lawyer for the victims who argued the case before the Supreme Court, said the ruling brings "long-overdue relief to more than 1000 victims of Iranian terrorism and their families, many of whom have waited decades for redress".
Jeffrey Lamken, the Iranian bank's lawyer, declined to comment.
The plaintiffs have waged a long legal battle seeking compensation for attacks they say Iran orchestrated. Congress inserted itself into the dispute by passing the law to help the plaintiffs obtain the Iranian funds.
The plaintiffs accused Iran of providing material support to Hezbollah, the Iranian-backed Shi'ite Islamist political and military group responsible for the 1983 truck bomb attack at the Marine compound in Beirut that killed 241 US service members.
They also sought compensation related to other attacks including the 1996 Khobar Towers truck bombing in Saudi Arabia that killed 19 US service members.
Caragh Fay, a lawyer representing the victims of the Beirut attack, said it could take from three months to a year for the funds to be dispersed to plaintiffs, depending in part on recommendations Sporkin first has to make to the judge.
