Iran says it only wants nuclear power for peaceful purposes
The US is circulating a discussion paper on possible further UN sanctions on Iran over its nuclear programme, says a Western diplomat at the UN.
The measures include expanding travel bans and asset freezes on people connected with the nuclear industry.
This comes despite the apparent acceptance by Iranian President Mahmoud Ahmadinejad of a deal.
This would allow Iran to send most of its low-enriched uranium abroad in return for research reactor fuel rods.
Washington has called on Iran to match its words with actions.
Last month, diplomats said Iran had informed the International Atomic Energy Agency (IAEA) that it did not accept the terms of the deal and had instead demanded a simultaneous exchange on its territory.
The US and its allies fear Iran is attempting to develop nuclear weapons. Iran insists its nuclear programme is entirely peaceful.
On Wednesday, the Western diplomat confirmed reports by the Reuters news agency that the US paper talks about expanding travel bans and asset freezes on those connected to Iran's nuclear industry, as well as financial measures such as placing restrictions on its central bank.
BBC world affairs correspondent
If President Ahmadinejad's comments are followed up by an Iranian approach to the IAEA stating that it is ready to negotiate on a uranium deal, then the whole picture would be changed.
Up to this point, Iran has said that it wants major changes to the proposed deal, which would make it unacceptable to the US and its allies.
If, on the other hand, he is saying this now as a way of undermining the current discussions on increasing sanctions on Iran, there would be no progress.
As always with Iran, it is difficult to assess its policy from one statement. But at least there is some hope.
In particular, it suggests targeting senior members of the Iran's Revolutionary Guards Corps (IRGC), which oversees Iran's nuclear programme and its strategic weapons.
The BBC's Barbara Plett, at UN headquarters in New York, says this is very much a preliminary stage. The US and its European allies on the Security Council - the UK, France and Germany - would have to eventually win agreement from Russia, which has been reluctant to pursue sanctions, and China, which openly opposes them.
The diplomat said there was also some talk about sanctions targeting the energy sector, but "it's very early days in terms of substantive talks among the six" which have been negotiating with Tehran over its nuclear programme.
"It's not a detailed text, it's just the beginnings of looking at what categories and measures might be pursued," said the diplomat.
The apparent policy shift in Iran also complicates things, our correspondent adds.
On Tuesday, President Mahmoud Ahmadinejad said Iran would have "no problem" sending most of its low-enriched uranium abroad so it could be processed into fuel, as envisaged by a deal struck in October between Iran, the IAEA and the so-called P5+1 - the US, Russia, China, UK, France plus Germany.
"We have no problem sending our enriched uranium abroad," he said.
"We say: We will give you our 3.5% enriched uranium and will get the fuel. It may take four to five months until we get the fuel."
"If we send our enriched uranium abroad and then they do not give us the 20% enriched fuel for our reactor, we are capable of producing it inside Iran," he added.
NUCLEAR FUEL CYCLE
Mined uranium ore is purified and reconstituted into solid form known as yellowcake
Yellowcake is chemically processed and converted into Uranium Hexafluoride gas
Gas is fed through centrifuges, where its isotopes separate and process is repeated until uranium is enriched
Low-level enriched uranium is used for nuclear fuel
Highly enriched uranium can be used in nuclear weapons
Our correspondent says the US, UK, France and Germany are now waiting to see if this is Iran's official position - suspicious that it is a tactic to stall for time.
A White House official earlier told the BBC: "If Mr Ahmadinejad's comments reflect an updated Iranian position, we look forward to Iran informing the IAEA."
The British Foreign Office also said that it "looked forward" to Tehran notifying the IAEA.
The French Foreign Minister, Bernard Kouchner, meanwhile suggested the Iranians were stalling and said he was "perplexed and even a bit pessimistic" about Tehran's offer.
His German counterpart, Guido Westerwelle, told reporters that Iran had to be "measured by its actions, not by what it says" and that it was up to Iran to "show an end to its refusal to negotiate".
Moscow gave a guarded response to the Iranian offer, with Foreign Minister Sergei Lavrov saying it would welcome Iran's acceptance of the scheme.
The UN Security Council has already imposed three rounds of sanctions against Iran in an effort to force it to stop all uranium enrichment activities and heavy water projects.