The deputy head of the UN nuclear agency has arrived in Tehran for talks on Iran's disputed nuclear programme.
Six world powers agreed to consider further sanctions against Iran during discussions on Wednesday.
This week Iran missed a deadline to reply to an offer of incentives to end its uranium enrichment programme, which could be used in arms manufacture.
France, the US and UK are pushing for new sanctions, but Russia says there is potential for more dialogue.
Correspondents say it is not clear that the visit of Oli Heinonen, the International Atomic Energy Agency's deputy head, is directly related to the incentives offer.
'FREEZE-FOR-FREEZE' OFFER
Three sets of UN sanctions have already been imposed on Iran after it defied successive Security Council ultimatums to freeze uranium enrichment.
The fact Iran is enriching uranium has led Western powers to fear it is seeking to develop an atomic bomb.
Tehran insists continuing the process is within its rights as an IAEA member and denies there is any intention beyond peaceful energy production.
'Clear response'
The threat of new sanctions came after a telephone conference on Wednesday between the five permanent members of the UN Security Council - China, France, Russia, the UK and US - and Germany.
EU foreign policy chief Javier Solana had given Iran's chief nuclear negotiator until last Saturday to respond to an offer of no further economic sanctions in return for an Iranian freeze on uranium enrichment.
On Tuesday, Iran delivered a one-page letter to Mr Solana's office, saying it was ready to provide a clear response "at the earliest possibility", but at the same time seeking a clear response to questions it has asked.
"Such mutual clarification can pave the way for a speedy and transparent negotiating process with bright prospect," the letter said.
The US administration said the letter appeared to be a stalling tactic.
"We are very disappointed that Iran has failed yet again to give... a clear answer," said US state department spokesman Gonzalo Gallegos.
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