The US Supreme Court on Wednesday ruled that almost $2 billion in frozen Iranian assets must be turned over to American families of people killed in the 1983 bombing of a U.S. 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 toward satisfying a $2.65 billion judgment won by the families against Iran in U.S. federal court in 2007.

Bank Markazi had challenged a 2014 ruling by the New York-based 2nd US Circuit Court of Appeals that the money, held in a Citibank trust account in New York, should be handed over to the American plaintiffs.

"We are extremely pleased with the Supreme Court's decision, which will bring long-overdue relief to more than 1,000 victims of Iranian terrorism and their families, many of whom have waited decades for redress," said Ted Olson, a lawyer for the victims.

Lawyers for the Iranian bank did not immediately respond to a request seeking comment.

The lawsuit was brought by more than 1,000 Americans who 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 American plaintiffs obtain the Iranian funds.

The plaintiffs accused Iran of providing material support to Hezbollah, the Iran-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.

The ruling, written by liberal Justice Ruth Bader Ginsburg, said the US Congress did not violate the separation of powers principle enshrined in the U.S. Constitution that gives specific authority to the government's executive, legislative and judicial branches.

Ginsburg wrote that the law was "no threat to the independence of the judiciary" because it did not apply simply to one case, but rather "multiple civil actions" against Iran filed by numerous plaintiffs. The law simply set a new legal standard and left to the courts to determine how that standard should be implemented, Ginsburg said.

As such, the law "does not transgress constraints placed on Congress and the president by the Constitution," Ginsburg wrote.

She also noted that courts generally give Congress and the White House greater leeway when it comes to foreign affairs.

Chief Justice John Roberts, a conservative, criticized the ruling, saying he believes Congress was "commandeering the courts to make a political judgment look like a judicial one." Liberal Justice Sonia Sotomayor joined his dissent.

The Obama administration, the US Senate and a legal group representing leaders of the House of Representatives all filed court papers backing the families.

The lead plaintiff in the case is Deborah Peterson, whose brother, Marine Lance Corporal James Knipple, died in the Beirut bombing.

"The Supreme Court's decision upholds the decisions of both Congress and the executive branch to hold Iran accountable for its actions supporting and sponsoring terrorism, and we are gratified that the court agreed that the law Congress enacted to provide relief to victims of terrorism complies with the Constitution," added Olson, the lawyer for the victims.

The ruling came during a delicate period in US-Iranian relations, following the January implementation of a landmark accord reached last year by the United States and five other world powers to lift economic sanctions in exchange for Iran accepting limits on its nuclear program.-Reuters