New York: Iran escalated its invective against the United States on Thursday over the court-ordered use of nearly $2 billion in seized Iranian central bank assets to compensate US victims of terrorist attacks overseas, calling the action “an outrageous robbery” and threatening unspecified retaliation.

The criticism of the United States, contained in a letter by Iran’s foreign minister to the UN secretary-general, was among the most strident yet in a steadily increasing display of anger from Iran over the US use of the assets, validated in an April 20 decision by the US Supreme Court.

The decision concerned compensation claims by more than 1,000 Americans — survivors and relatives of people killed in attacks that the US authorities have attributed to Iranian operatives — despite Iran’s denials of responsibility. The attacks include the 1983 truck bombing of a Marine base in Beirut and a 1996 truck bombing in Al Khobar, Saudi Arabia.

Iran had previously called the Supreme Court decision a form of theft and had suggested it might sue the United States in the International Court of Justice to block the ruling from taking effect. But Thursday appeared to be the first time Iran suggested it might be planning a reciprocal response.

“The entire court proceedings which led to the recent ruling has been fake and phoney and a travesty of justice in every sense of law, jurisdiction, merit, fact and process,” Foreign Minister Mohammad Javad Zarif wrote in the letter to Secretary-General Ban Ki-moon, which was released by Iran’s UN Mission.

“The Islamic Republic of Iran holds the United States government responsible for this outrageous robbery, disguised under a court order, and is determined to take every lawful measure to restore the stolen property and the interest accrued to it from the date it was blocked by the United States,” Zarif wrote.

The letter denounced what Zarif described as a history of US arrogance towards Iran and other sovereign states through the use of US courts to seek collection of civil judgements from them, which he called a “dangerous practice of defying international law”.

He also recounted a litany of Iran’s own grievances with the United States, beginning with what Zarif called the CIA-supported overthrow of the government in 1953, US support for the former shah’s secret police, US backing for Saddam Hussain in the Iran-Iraq war, and the shooting down of an Iranian jetliner in 1988 by US naval forces in the Gulf.

“It is in fact the United States that must pay long overdue reparations to the Iranian people for its persistent hostile policies,” Zarif wrote.

“The Islamic Republic of Iran reserves the right to take appropriate lawful action, including necessary and proportionate countermeasures, to restore and protect the rights of the Iranian people against such persistent unlawful conduct by the United States,” he wrote.

There was no immediate comment from the State Department on the letter, which Zarif asked Ban’s office to circulate as a document of the UN General Assembly and Security Council.

Authorship of the letter in Zarif’s name was significant in part because he has been among the staunchest defenders of seeking improved relations with the United States.

Zarif, along with Secretary of State John Kerry, were among the lead architects of the landmark agreement reached in July that curtailed Iran’s nuclear activities in exchange for eased economic sanctions. Both sides expressed hope that it could lead to improved diplomatic ties.

On Wednesday, President Hassan Rouhani of Iran, who was elected in 2013 partly on promises of achieving that nuclear agreement, castigated the United States over the Supreme Court decision, calling it a “flagrant theft and legal disgrace”.