The US says Iran is - at the least - keeping its nuclear options open
Iran is moving closer to being able to make a nuclear bomb, the US envoy has told a meeting of the International Atomic Energy Agency (IAEA) in Vienna.
Glyn Davies told the meeting Iran was nearly or already in possession of enough low-enriched uranium to produce a bomb, if it was further enriched.
Iran denies seeking anything beyond a civilian nuclear power programme.
It has now presented a package of new proposals to the group of six world powers negotiating over its programme.
Diplomats from those powers - the five permanent members of the UN Security Council plus Germany - will hold a conference call later in the day to discuss the proposals.
There were no plans to issue a statement afterwards, an official said.
The negotiations are taking place as the IAEA - the UN nuclear watchdog - holds a week-long meeting in Vienna.
'Dangerous and destabilising'
"We have serious concerns that Iran is deliberately attempting, at a minimum, to preserve a nuclear weapons option," US envoy Mr Davies told the 35 nations on the IAEA board of governors.
Peter Biles, world affairs correspondent, BBC News
There remains deep anxiety in the West about Iran's refusal to answer all the outstanding questions related to its nuclear programme.
Analysts say there is fairly broad consensus that Iran has conducted a process of "weaponisation". But at precisely what stage does Iran constitute a threat to the international community?
Israel is said to be arguing that a red line is crossed as soon as Iran has a "significant uncontained nuclear capability". In other words, it need not be the actual development of a nuclear bomb.
So the six major powers dealing with this - the permanent members of the UN Security Council, plus Germany - will be keen to know when Iran reaches "breakout capability", the wherewithal to produce a weapon.
"Iran is now either very near or in possession already of sufficient low-enriched uranium to produce one nuclear weapon if the decision were made to further enrich it to weapons-grade."
He said this moved Iran "closer to a dangerous and destabilising possible break-out capacity" - the point at which it could create an atomic bomb.
Tehran has now handed over a new set of proposals for handling the issue to the six powers.
Iranian Foreign Minister Manouchehr Mottaki on Tuesday said that Tehran would be willing to launch a new round of talks "in the framework" of the proposals.
But in Vienna Iran's envoy to the IAEA, Ali Asghar Soltanieh, also told the board of governors that the agency had long been faced with false accusations about Iran from the US.
"The world is observing curiously whether or not this [US] administration follows the same trend as the Bush administration - pursuing hostile political confrontation, using fabricated baseless allegations," he said.
Some observers believe Tehran might just be stalling for time, reports the BBC's world affairs correspondent, Peter Biles.
President Mahmoud Ahmadinejad is due to attend the UN General Assembly in New York later this month, and Iran's willingness to co-operate internationally is now under greater scrutiny than ever, he says.
The head of the IAEA, Mohamed ElBaradei, has said in the past that the threat posed by Iran is exaggerated.
But on Wednesday he urged Tehran to accept the US offer of dialogue, adding that unless both countries talked to each other - not at each other- then no progress would be made.
"The US is making an offer without preconditions and on the basis of mutual respect," Mr ElBaradei told reporters before the IAEA convened.
The IAEA chief has sounded a note of impatience with Iran
"The offer by the US is an offer that should not be refused, that cannot be refused, because it has no conditions attached to it. And I hope [the] response will be positive."
In its latest report, the IAEA said a visit to Iran's Natanz plant in August had noted a reduction in the number of centrifuges used to actively enrich uranium.
But it also accused Tehran of a lack of co-operation with the IAEA on Western intelligence allegations of weaponisation.
Iran is continuing to enrich uranium in defiance of the UN Security Council, saying it has a right to a nuclear power programme.