The European Union also used a board meeting of the U.N.'s International Atomic Energy Agency to pile pressure on Iran to stop obstructing an IAEA investigation into suspected atom bomb research by Tehran, which denies the charge.
Washington and its Western allies were signaling their determination that the Islamic state must give the IAEA access to sites and documents, regardless of broader talks between Iran and world powers that resumed last week.
Iran's envoy to the Vienna-based U.N. agency hit back, saying the allegations over his country's nuclear work were "baseless" and suggested the IAEA, not Tehran, was to blame for the failure so far to revive the stalled inquiry.
Some diplomats say Iran is using its meetings with the IAEA merely for leverage in negotiations with world powers which, unlike the U.N. agency, have the power to ease sanctions that they have recently tightened on the major oil producer.
U.S. envoy Joseph Macmanus accused Iran of "provocative actions", particularly the installation of advanced centrifuges that would enable it to speed up its uranium enrichment.
"We are deeply concerned with what appears to be Iran's unwavering commitment to deception, defiance, and delay," Macmanus told the IAEA's board of governors.
Western countries fear Iran is enriching uranium to develop the capacity to build nuclear weapons and have imposed several rounds of sanctions. Iran says the program is legitimate and intended for purely peaceful purposes.
Israel says Tehran is secretly trying to develop a nuclear weapon and has threatened pre-emptive strikes if it deems diplomacy ultimately futile.
The Vienna-based IAEA has been trying for more than a year to persuade Iran to give it the access it says it needs for its investigation, so far without progress.
EU sees Iran "procrastination"
Iran has refused IAEA requests to visit the Parchin military site, where inspectors suspect explosives tests relevant for nuclear weapons development took place, possibly a decade ago.
"Iran is inviting further isolation, pressure and censure from the international community ... until it meets its obligations and addresses the board's concerns," Macmanus said.
He suggested the IAEA may need to change its tactics if the talks with Iran remain unproductive, but did not elaborate.
The EU told the IAEA board that it "considers ... Iran's procrastination to be unacceptable".
Iran says it first needs to agree with the IAEA on how the inquiry is to be conducted before allowing any Parchin visit.
"We are committed to continue our dialogue with the IAEA but at the same time we can't give a blank check" because of Iran's national security concerns, Iranian Ambassador Ali Asghar Soltanieh told reporters on the sidelines of the board meeting.
In the Kazakh city of Almaty last week, six world powers resumed talks with Iran aimed at finding a diplomatic settlement to a decade-old dispute that threatens to trigger a new Middle East war. The only progress was an agreement to hold more talks.
The United States, China, France, Russia, Britain and Germany offered modest relief from economic sanctions in return for Iran scaling back its most sensitive nuclear activity.
Iran called the talks a potential "turning point" but Western officials were more cautious, merely describing the meeting as "useful".
Macmanus said: "I must make clear that we will not accept further delay by Iran in regard to implementing its IAEA obligations and that the separate (big power) diplomatic process cannot be a substitute for such implementation."
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