The Vienna-based International Atomic Energy Agency (IAEA) on Friday imposed an October 31 deadline on Iran to prove it is not secretly developing atomic weapons, even though Tehran denies it is doing so.
"A lot of pressure has been put on Iran for the last several years to no great effect. If the rest of the world joins in so that it is the United States, Russia and the Europeans together, that would start to hurt," William Hopkinson of London's Royal Institute of International Affairs told AFP.
"(The IAEA is) sending a very powerful message to Iran that they need to cooperate fully and immediately and to show complete transparency," IAEA director general Mohamed ElBaradei told reporters in Vienna.
ElBaradei has warned Iran the IAEA could declare it to be in non-compliance with the global nuclear Non-Proliferation Treaty (NPT) if it failed to answer all the agency's questions on its nuclear activities.
That could lead to the issue being referred to the UN Security Council, which has the power to impose punishing sanctions on the Islamic republic.
Analysts said that Iran is an different situation than North Korea which the IAEA took in February to the Security Council over non-compliance, and has withdrawn from the NPT.
North Korea already has some nuclear bombs while Iran still wants to raise its military capabilities to match Israel, which has nuclear weapons, Christopher Aaron, editor of Jane's Intelligence Review said.
And even if Israel were not a threat, Iranian leaders fear the possibility of fellow Muslims, the Taliban, one day taking over or having influence in Pakistan, which also has nuclear weapons, Hopkinson said.
"There are many reasons why Iran would wish, even if reformers came to power there, to have nuclear weapons," he said.
Iran has angrily hit back at the UN nuclear watchdog for imposing a deadline.
Tehran's ambassador to the body, Ali Akbar Salehi, told the official news agency IRNA that: "The Western group in the (IAEA) board of governors, in line with their political goals, have made illegitimate, illegal and impractical requests from Iran."
The ultra-hardline Jomhuri Eslami newspaper wrote that Iran "should not pay any attention to the US, the Europeans and international organisations... and accept that the right path is the one that the North Koreans have chosen."
Hopkinson said he thought the only possible way to pressure Iran was through UN sanctions, with all the "major players" lined up against Tehran.
Washington had lobbied hard during five days of closed-door talks by the IAEA's 35-nation board of governors to get the resolution adopted.
France and Germany supported the US in setting the deadline on Iran, in sharp contrast to their opposition earlier this year to the US-led invasion of Iraq, which was also alleged to have had weapons of mass destruction.
It was unclear in Vienna how strongly Russia supported the IAEA resolution, which was adopted without a vote.
Russia's Foreign Minister Igor Ivanov on Friday said Moscow was committed to attempts by the UN nuclear watchdog to prevent Iran from developing nuclear weapons.
But he said Russia still wanted to continue its 800-million-dollar nuclear cooperation programme to build Iran's first nuclear plant.
In any case, Iran is close to getting the bomb, according to Israeli intelligence.
Major General Ze'evi Farkash, head of Israeli military intelligence (AMAN), told the London-based Jane's Intelligence Review that once Iran "has the ability to produce enough enriched uranium, we estimate that the first bomb will be constructed within two years, that is the end of 2006 or the beginning of 2007.
"We, though, are relating to the earlier year 2004. Israel views that as the point of no return, that is the moment when all the necessary knowledge will be within the grasp of Iranian scientists," Farkash said.