The United States, Britain, France and Germany said they were fed up with Iran's alleged evasion of UN inspectors and obfuscation of its nuclear activities, demanding Tehran immediately come clean.
The joint statement by the grouping calling itself the quad follows a report by the International Atomic Energy Agency (IAEA) saying Iran had sharply increased its stockpile of enriched uranium enough, if refined further, to make six atom bombs.
"Our patience has been long, but it is not unlimited," UK Ambassador Corinne Kitsell said on behalf of the quad in a letter to the IAEA.
The statement marked an escalation of Western rhetoric on the disputed program which Tehran insists is peaceful but could be the focus of a potential military strike by arch-foe Israel after the rivals have battled throughout the Mideast for over a year.
"Until now, Iran has made its choice. Let us be clear: unless Iran changes course, it will force the (IAEA) Board to make its own choice. Time is not on Iran’s side," Kitsell added.
"Iran has instead chosen a path of escalation, obfuscation, and delay. Iran must be held to account if it continues along this path."
At issue is the potential designation of Iran by the IAEA board as "noncompliant" in the watchdog's next quarterly meeting in June, which could trigger a referral of the issue to the UN Security Council in an uncomfortable move for the isolated theocracy.
The United States on Tuesday said Iran's accelerating nuclear program is imperiling world peace and either aims to get Tehran closer to a bomb or extort the world to extract concessions.
The European Union urged Iran to immediately halt and reverse its accelerating nuclear program.
They were responding IAEA Director General Rafael Grossi's report to the body's board the previous day that Iran's uranium stock refined to up to 60% purity grew by 92.5 kilograms (kg) in the past quarter to 274.8 kg.
That amount, according to an IAEA yardstick, is enough in principle for six nuclear bombs.
Iran’s ambassador to the IAEA said on Wednesday that European parties to a now mostly lapsed 2015 international deal over Iran's nuclear program lacked the standing to trigger a so-called snapback of UN sanctions on Tehran over non-compliance.
Such a move would deal a further blow to Iran's heavily sanctioned economy as economic malaise and political polarization have deepened.
