The International Atomic Energy Agency is expected to discuss a resolution next Tuesday calling on Iran to respond urgently and completely about its nuclear program.
Western diplomats say a report on Iran's nuclear program to be presented to the IAEA board of governors on Monday is cause for significant concern.
The diplomats, who spoke on condition of anonymity, say the Iranians have changed their story five or six times, which the diplomats say proves Iran has lied about its nuclear ambitions.
Western diplomats also say Iran has not fully cooperated with IAEA inspectors. They say the agency was denied access to one crucial facility until just recently, and when they got in it was clear there had been considerable modifications.
Sources say analysis of samples taken at the facility will not be ready until October and the modifications could affect the accuracy of the results.
The (IAEA) Board of Governors is expected to debate a resolution on Iran on Tuesday. Any resolution is expected to call on Iran to explain what happened at the facility so that additional work can be carried out by the IAEA scientists.
But diplomats say the resolution will fall short of saying Iran is guilty of non-compliance with IAEA rules, wording which could send the matter to the United Nations Security Council.
Ambassador Kenneth Brill, head of the U.S. delegation at the IAEA, told reporters Friday he is convinced the board will take the appropriate action.
"I think its fair to say that the overall majority of board members want to see the IAEA get to the bottom of the Iran nuclear file," he said. "They want to see Iran enhance its cooperation and they want the agency to get the answers to all the questions that are outstanding and I would note that there are more questions today than there were when the agency began its work."
The United States believes Iran is working on a secret nuclear weapons program.
IAEA officials have visited Iran five times since June. They say the country has a large and sophisticated nuclear program. And restrictions on IAEA activities in Iran raise concerns about how that program might be used.