US ambassador to the Vienna-based International Atomic Energy AgencyKenneth Brill told reporters that Iran's refusal to fully cooperate with the agency "fits a long-term pattern of denial and deception that can only be designed to mask Iran's military nuclear program."
He was commenting after an IAEA report released on Tuesday charged that agency inspectors had found more traces in Iran of highly enriched uranium that could be bomb-grade.
The IAEA also reported that Iran, which claims its nuclear program is for peaceful, civilian purposes, has admitted to importing parts for sophisticated P-2 centrifuges for enriching uranium, going back on claims that it had made the parts domestically.
"Almost two years after the IAEA became aware of Iran's covert nuclear program, and fully one year after the discovery of Iran's attempts to conceal their work at the Kalaye Electric Company (in Tehran), delayed access, inconsistent stories and unanswered questions continue to be the hallmark of Iranian cooperation with the agency," Brill said.
"Even a disinterested observer must now ask, what is it that the Iranians are so intent on hiding," Brill said.
The IAEA report is to be submitted to the agency's 35-nation board of governors on June 14.
The United States has called for the IAEA, which has been investigating the Iranian program since February 2003 after being alerted to it in August 2002, to refer the Islamic Republic to the UN Security Council for possible international sanctions.
In Tehran, Iran's top nuclear negotiator Hassan Rowhani said Wednesday that Iran had "no secret nuclear activities".
Rowhani said: "Iran's nuclear dossier is on the way to being sorted out and there is nothing very important that is pending."
But Brill said: "Iran is still stalling, providing last-minute declarations and contradicting earlier definitive statements. The IAEA continues to find new, incriminating evidence of undeclared activity."
"The question is how long the (IAEA) board of governors and the international community will tolerate this," he said.
But diplomats and experts said they expected nothing would happen until after the presidential election in November in the United States, and a change in US policy, since Washington does not have a "smoking gun" to prove Iran is making nuclear weapons and so can not convince the IAEA board to back it in taking the issue to the Security Council.
"It looks like this meeting in June is not going to produce much," Gary Samore, a London-based non-proliferation expert, told AFP.
"The United States is stymied unless the IAEA can come up with some devastating revelation that Iran is lying or hiding something," he said.
He said the Iranians may even be emboldened with their success in putting the IAEA off to resume the enrichment of uranium, something they voluntarily suspended in order to build confidence with the international community.
But Samore said the European big three -- Britain, France and Germany -- which struck an agreement last October for Iran to suspend enrichment "have warned the Iranians very strongly against this."
A Western diplomat close to the IAEA said such a move by Iran "would spark a crisis with the EU-3 and others and I don't think that's a price Tehran wants to pay."
Highly enriched uranium can be used for fuel in reactors but also the explosive in atomic bombs.
Another diplomat said nothing less than the international non-proliferation regime was at stake in Iran.
The diplomat said getting to the bottom of the Iranian program was "difficult but crucial for non-proliferation and for the IAEA."
"If progress is slow, there will come a time when the Europeans will have to reflect on their policy," the diplomat said .
But the diplomat said this time was not imminent.
"Even if Iranian cooperation is not 100 percent, progress (since IAEA investigations began 15 months ago) has still been considerable," in uncovering Iranian nuclear activities, the diplomat said.