The International Court of Justice on Wednesday will give its decision on a bid by Iran to recover $2 billion in frozen assets that the United States says must be paid to terror victims.
The US Supreme Court ruled in 2016 that Iran must give the cash to survivors and relatives of victims of attacks blamed on Tehran, including the 1983 bombing of a US Marine barracks in Beirut.
Iran said the US decision breached 1955 Treaty of Amity with the United States, an agreement signed before Iran's 1979 Islamic Revolution severed relations between the countries.
At the last hearing on Iran's appeal in October at the Hague-based tribunal, Washington said Iran has "unclean hands" and that its alleged support for terrorism should disqualify the case from being heard.
The ICJ is the top court of the United Nations and was set up after World War II to resolve disputes between member states. Its rulings are binding and cannot be appealed, but it has no means of enforcing them.
Wednesday's ruling threatens to throw more fuel on the fire after a decision in October when the court ordered the US to lift sanctions on humanitarian goods for Iran.
The United States announced hours after that decision it was pulling out of the Treaty of Amity, upon which Iran had also based the sanctions case.
Tensions between Tehran and Washington are already high around the anniversary of the 40th anniversary of the Iranian revolution.
- 'Bad faith' -
Relations have been strained since US President Donald Trump's decision to pull out of an international nuclear deal with Iran and reimpose sanctions.
Iran first lodged the case on the frozen funds in June 2016, accusing Washington of breaking the decades-old bilateral treaty dating from the time of the Shah, who was deposed in the revolution.
Tehran said the United States had illegally seized Iranian financial assets and those of Iranian companies.
In October, Richard Visek, a US State Department legal official, told the ICJ that "Iran comes to the court with unclean hands -- indeed, it is a remarkable show of bad faith."
US Secretary of State Mike Pompeo had added at the time that "we owe it to our fallen heroes, their families, and the victims of Iran’s terrorist activities to vigorously defend against the Iranian regime’s meritless claims... in The Hague."
The US Supreme Court ruled in April 2016 that the $2 billion in frozen Iranian assets should be paid to about 1,000 survivors and relatives of those killed in attacks blamed on the Islamic Republic.
As well as the Beirut Marine barracks attack, in which 241 soldiers were killed, these also included the 1996 Khobar Towers bombing in Saudi Arabia.
The timing of the US court ruling had been particularly sensitive as it was just a year after the landmark nuclear deal with world powers which led to the unblocking of other frozen funds.