The United States on Monday urged the UN's top court to throw out a bid by Iran to lift sanctions imposed by President Donald Trump, saying Tehran remained a "grave threat" to global security.

Iran brought the claim to the International Court of Justice (ICJ) in 2018, alleging that Trump's decision to pull out of a nuclear pact with Tehran breached a 1955 treaty of friendship between the two countries.

But US lawyers argued Monday that the Hague-based tribunal does not have jurisdiction in the case, accusing Iran of abusing the decades-old "Treaty of Amity" to try to get sanctions relief for its struggling economy.

"Iran's efforts to shoehorn this dispute into a legal instrument not intended for the purpose... are entirely without merit," Marik String, Acting Legal Adviser of the US State Department, told the court via videolink.

"We respectfully request dismissal of Iran's case."

The US official said the sanctions were necessary because of Iran's "destabilising nuclear programme", its ballistic missile activity, Tehran's alleged support for "terrorism and regional destabilisation."

"The measures that Iran challenges remain critical to US efforts to address national security threats posed by Iran," String said, adding that Washington had "long considered Iran's conduct to present a grave threat."

Iran will have the chance to answer on Wednesday, with hearings lasting until next Monday.

The current hearings deal only with whether the ICJ has jurisdiction. The tribunal will only move on to the merits of the case if and when it decides it is allowed to deal with it.

A decision on jurisdiction by the ICJ, which was set up after World War II to rule in disputes between nations, could take several months, while a final ruling would take years.

Iran says the sanctions brought back by the Trump administration breach the 1955 "Treaty of Amity"
Iran says the sanctions brought back by the Trump administration breach the 1955 "Treaty of Amity" Iranian Presidency / Handout

Tehran won an early victory in October 2018 when the ICJ ordered sanctions on humanitarian goods to be eased as an emergency measure while the overall lawsuit is dealt with.

The US responded by formally ending the Treaty of Amity, agreed when Iran was ruled by the Western-oriented shah.

The ICJ is also dealing with a separate case over Tehran's bid to unfreeze $2 billion in assets frozen in the United States.

The 2015 nuclear deal -- -- involving the five permanent members of the UN Security Council -- Britain, China, France, Russia and the United States, plus Germany -- has hung by a thread since the Trump pulled out

The accord promises Iran sanctions relief in exchange for curbing its nuclear programme, but Tehran has stepped up nuclear activities since last year after the US reimposed sanctions.

Washington has reimposed sanctions on Iran and companies with ties to it, notably hitting Iran's vital oil sector and central bank, while major global firms halted their activities in Iran.

In the latest development, the head of UN's nuclear watchdog said Monday that inspectors would "in a few days" visit the second of two sites in Iran where undeclared nuclear activity may have taken place in the early 2000s.

Iran's refusal to allow the International Atomic Energy Agency (IAEA) access to the two sites had caused a diplomatic row for months.