Tensions between Iran and the UN nuclear watchdog have repeatedly flared in recent years, with relations being further strained in the wake of a 12-day war in June that saw Israeli and US strikes on key Iranian nuclear facilities.

In its latest report, the International Atomic Energy Agency (IAEA) said it "has lost continuity of knowledge in relation to previously declared inventories of nuclear material in Iran at the affected facilities", including its stockpile of near-weapons-grade uranium.

According to the IAEA, Iran had an estimated 440.9 kilograms enriched to up to 60 percent as of June 13 but has been unable to verify it.

The agency said Iran is the only country without nuclear weapons to enrich uranium to 60 percent, close to the 90 percent threshold required for a bomb.

Iran's near-bomb-grade uranium stockpile was "a matter of serious concern", it said in its report, adding that its verification after five months of no access to it "is long overdue".

"It is critical that the agency is able to verify the inventories of previously declared nuclear material in Iran as soon as possible in order to allay its concerns and ensure compliance with the (Non-Proliferation Treaty) NPT safeguards agreement regarding the possible diversion of declared nuclear material from peaceful use," the report added.

Western powers and Israel have long accused Iran of seeking to acquire nuclear weapons, a claim Tehran denies.

Nuclear talks between Iran and world powers are currently stalled.

A now-defunct 2015 deal curbed Tehran's nuclear programme in exchange for sanctions relief.

Trump unilaterally withdrew the United States from a landmark 2015 deal with Tehran during his first term as president.