The U.N. nuclear agency responsible for probing whether Iran has worked on a nuclear bomb depends on the United States and its allies for most of its intelligence, complicating the agency's efforts to produce findings that can be widely accepted by the international community.

Much of the world looks at U.S. intelligence on weapons development with a suspicious eye, given American claims a decade ago that Iraq had developed weapons of mass destruction. The U.S. used those claims to justify a war; Iraq, it turned out, had no such weapons.

The International Atomic Energy Agency insists that it is objective in evaluating Iran's nuclear program and that its information comes from a wide range of sources and is carefully vetted. But about 80 percent of the intelligence comes from the United States and its allies, The Associated Press has been told.

Two IAEA officials, who gave the 80 percent figure, told The AP that the agency has been forced to rely more and more on information from Iran's harshest critics — the U.S., Israel, Britain, France and Germany — because Tehran refuses to cooperate with international inspectors.

Their evaluation appeared to be the first in percentage terms. The officials demanded anonymity because they are not authorized to release classified information.

All five nations accuse Iran of having worked on nuclear arms, with Israel and the U.S. not ruling out force as a last resort if diplomacy fails to curb programs that Tehran could use for such weapons.

France and Germany refrained from joining the Iraq invasion, insisting U.S. intelligence about Saddam Hussein's purported weapons program was inconclusive.

Intelligence services of other nations, such as Pakistan, China or Russia, also collect information on Iran. But they are compromised by the fact that their governments or individuals provided the equipment or knowledge in the past that allowed Iran to develop its nuclear program.

Today, they are reluctant to pass on what they know to the agency for political reasons — they want to be viewed as above the fray. They also view the IAEA more as technical organization and less as the U.N.'s nonproliferation watchdog, a role the agency has increasingly assumed with its Iran probe.

That leaves the U.S. and its allies as the IAEA's main intelligence sources.

Critics invoke the Iraq fiasco to warn that the information on Iran provided by Tehran's adversaries may be at best inaccurate and at worst spin, meant to pave the way for possible attack.

"Memories of the failure and tragic mistakes in Iraq are not taken sufficiently seriously," Hans Blix, a former IAEA chief, told reporters in Dubai in March.

"There is no evidence right now that suggests that Iran is producing nuclear weapons," said Blix, who headed the team that combed Iraq in the vain search for weapons of mass destruction.

Tehran has played on the credibility gap left by Iraq as it insists it is not interested in nuclear weapons, even as it pursues a program that is near the ability to make them.

Asked about the information on which the accusations against Iran are based, Ali Asghar Soltanieh, Iran's chief delegate to the IAEA, urged the world to pay heed to "lessons learned from Iraq" in comments to the AP.

In a November 2011 report that summarized its suspicions, the IAEA said that all its intelligence on Iran "has been carefully and critically examined." But its ability to vet information has been hampered by Iran's refusal to give experts access to sites, documents and people the IAEA suspects of involvement in possible weapons research.

Such access effectively ended more than five years ago when Tehran announced it had answered all questions which it is obliged to under an agreement worked out with the U.N. agency. That has left the agency mostly dependent on outside intelligence — and has reduced its means of crosschecking that intelligence.

A cable from the U.S. mission to the agency citing IAEA chief Yukiya Amano telling mission officials that he is "solidly in the U.S. court" on Iran — published by Wikileaks in 2009 — also helps those arguing that the case against Tehran could be overblown.

International concerns about Iran's nuclear intentions date to the fall of the Shah in 1979. Those concerns resurfaced shortly before the 2003 Iraq invasion when U.S. spy satellites verified claims by Iran's exiled opposition that Tehran was assembling a uranium enrichment program at Natanz, in central Iran.

Six years later, Iran acknowledged to the IAEA that it was building a fortified underground site at Fordo, southwest of Tehran, to enrich uranium. It did so a few days after the U.S. shared intelligence with the IAEA on its existence.

But those revelations in themselves do not prove that Iran is interested in nuclear arms.

Although uranium enriched to weapons-grade is used for the core of nuclear warheads, the Iranians have so far enriched only to grades suited for nuclear fuel, medicine and science.

Iran insists it has no intention of making weapons and asserts it, like Japan and other non-nuclear arms states, is within international rights to enrich.

In its November 2011 report, the IAEA said that Iran appeared to have conducted high explosives testing and detonator development to set off a nuclear charge, as well as computer modeling of a core of a nuclear warhead.

It also cited alleged preparatory work for a nuclear weapons test, and development of a nuclear payload for Iran's Shahab 3 intermediate-range missile.

The agency says some such work may be continuing. Without a smoking gun, Iran and its supporters have challenged the IAEA to go public with its intelligence so the world can examine the allegations.

But the agency is obligated to countries supplying it with information to maintain secrecy. IAEA officials also fear that revealing too much might tip off Tehran and allow it to hide activities under investigation.

Hence, assessments about Iran's intentions come down to a matter of trust — something many countries are unwilling to buy into after the Iraq debacle.

Gary Samore, the White House's top adviser on weapons of mass destruction until January, says only a "couple of outliers, like Venezuela and Cuba" doubt that Iran is seeking nuclear weapons capability.

"I can't recall talking to any foreign government officials who believe that Iran's program was peaceful," he told the AP, dismissing public statements to the contrary from critics of Washington as politically motivated.

Nevertheless, public support for Iran remains strong, particularly among the 120 countries that call themselves nonaligned. Many are receptive to Iranian arguments that Western pressure on Tehran is a tactic to keep lucrative nuclear technology out of their hands.

In Tehran last year, nonaligned countries directly challenged the Security Council's position on Iran's nuclear enrichment, backing the Iranian insistence that the program is peaceful.

Russia is a U.S. partner in trying to curb Iran's enrichment program. But only after Moscow expressed unhappiness with what it saw as the agency's dependence on intelligence from the U.S. and its allies last year did the agency start to to share some — but not all — of the intelligence it gets with a Russian expert who reports to the Kremlin.

Reflecting indirect distrust of that intelligence, Russia's Interfax news agency last year quoted Russian Deputy Foreign Minister Sergei Ryabkov as saying Moscow sees "no signs that there is a military dimension to Iran's nuclear program."

Even some experts who are skeptical of Iran question the IAEA's heavy reliance on limited sources of information.

Robert Kelley, a former senior IAEA official, describes agency claims of continued Iranian weapons work as "sketchy."

Kelley, who was part of the 2003 IAEA inspection team in Iraq, says that Iran may indeed have an ongoing weapons program. But he also suggests that the U.N. agency may be jeopardizing its impartiality "by constructing accusations based upon anonymous sources that are almost a decade old" and relying on information "clearly coming from known sources hostile to Iran."

"Remember the lessons of 2003," he told the AP.