Muhammad al-Baradai, head of the International Atomic Energy Agency, said on Monday that Iran cannot withhold full cooperation indefinitely.
He said: "We still have a central issue, and that is whether Iran has declared all its (uranium) enrichment activities.
"The way they have been engaging us on this issue has been less than satisfactory."
The United States has long accused Iran of trying to build a nuclear weapon under cover of a civilian atomic energy programme. Tehran insists it is only interested in generating electricity.
Diplomats said the United States would be pushing at Vienna talks, expected to last at least several days, for the IAEA to set Iran a deadline to cooperate fully.
Al-Baradai said any deadline would be a matter for the member states to decide, but added: "I think everybody would like to see this issue brought to a close in the next few months... because we cannot go on for ever."
"We still have a central issue, and that is whether Iran has declared all its (uranium) enrichment activities. The way they have been engaging us on this issue has been less than satisfactory"
He highlighted concerns over the detection of traces of low-enriched and highly-enriched uranium at nuclear sites in Iran, and over Tehran's work with advanced P2 centrifuges.
"These are two issues where we need accelerated and proactive cooperation on the part of Iran," he said.
Centrifuges are used in the process of enriching, or purifying, uranium for use in an atomic reactor or in a nuclear weapon.
Delegates at the meeting will consider a joint draft resolution from France, Germany and Britain rebuking Iran for lax cooperation with the UN nuclear watchdog. But diplomats said Washington wanted to go further.
"The Americans want a deadline," a diplomat from one of the 35 nations on the IAEA board said. "A deadline would be to keep the pressure on Iran."
Another diplomat said a deadline could be used to force Iran to finally keep some of the promises it made to the Europeans in October 2003, when Tehran agreed to suspend uranium enrichment activities in exchange for peaceful atomic technology.
Iran's president says Tehran's
nuclear programme is peaceful
Washington would also like a "trigger mechanism" that would call for the board to report Iran to the UN Security Council for possible sanctions if its cooperation remains sluggish.
In September 2003, the IAEA passed a resolution setting a 31 October deadline for Iran to submit a complete declaration of its nuclear programme. Tehran submitted the declaration on time, although it was later shown to be incomplete.
Last week, the European trio circulated a toughly worded draft resolution that "deplores" Iran's failure to fully cooperate with the IAEA and urged Tehran to urgently "resolve all outstanding questions".
Iranian negotiators are pushing the Europeans to remove the word "deplores" and generally soften the text, which already has the support of most of the 35 board members, diplomats said.
The IAEA's rebuke to Iran comes after the country's ambassador to Jordan accused Israel of being behind international concerns about it's nuclear programme.
George Bush says Iran wants to
build a nuclear bomb
Speaking in Amman on Sunday, Muhammad Irani said Israel will suffer a "painful" response if it attacks Iranian nuclear installations.
He added that Israel opposes Iran's "support for the oppressed Palestinian people".