Controversy increases as world leaders consider new sanctions after Tehran dodges nuclear question.
The UN atomic watchdog's deputy director general, Olli Heinonen, arrived in Tehran on Thursday for a two-day visit to discuss the nuclear controversy, a source in the Iranian atomic energy organsation told AFP.
The visit comes as the six world powers agreed Wednesday to consider new sanctions on Iran after Tehran, according to the United States and Britain, gave an ambiguous answer to their latest demand to freeze key nuclear work.
Washington and London said the diplomatic P5+1 group -- which includes fellow permanent UN Security Council members China, France, and Russia as well as partner Germany -- agreed Wednesday it had "no choice" but to act.
The United States said the move enjoyed support from Moscow and Beijing -- which have resisted taking a harder line on Iran -- but China had no immediate reaction and Russia's UN ambassador said he was unaware of such consensus.
"It may well be that in the course of those discussions some members of the six raised the issue of the sanctions," Russia's envoy Vitaly Churkin said.
"But to the best of my knowledge there has been no firm agreement or understanding or concerted work in this regard."
Earlier, top diplomats from the group discussed the stand-off by conference call in the wake of Iran's reply Tuesday to a rewards package for freezing uranium enrichment, said US State Department spokesman Gonzalo Gallegos.
They "have agreed that we have no choice but to pursue further measures against Iran as part of this strategy," Gallegos said after the call, which also included European Union foreign policy chief Javier Solana.
"Given the absence of a clear positive response from Iran and its failure to meet the deadline set by the UNSCR 1803, the P5+1 are discussing the next steps in the UNSC and beginning to consider the possible outlines of another sanction resolution," Gallegos read to reporters from a written statement.
The powers "have agreed that, while informal contacts between Mr Solana and Mr (Iranian negotiator Saeed) Jalili will continue, we now have no choice but to pursue further sanctions against Iran, as part of our dual-track strategy," British junior foreign minister Kim Howells said in a statement.
Asked whether there was agreement among the six to proceed to the drafting of a new sanctions resolution, France's UN deputy ambassador Jean-Pierre Lacroix replied: "Our objective is not sanctions for the sake of sanctions."
"We have to resort to the Security Council (sanctions) if we don't see there's any possibility to enter into a dialogue," he noted. "But we are getting closer to the point where we will make that determination."
Germany warned that Iran's reply was "insufficient" and urged a negotiated solution to the dispute over Western charges, denied by Iran, that Tehran's nuclear program conceals an atomic weapons quest.
"If Iran does not choose this path, the UN Security Council will be referred to once again," German Foreign Minister Frank-Walter Steinmeier said.
While Gallegos called Tehran's reply to the package "a stalling tactic," Churkin said: "We would have preferred a clear yes. But it is more complicated than that."
Churkin said that the Group of Eight wealthy industrialized countries, including some of Iran's top trading partners, would discuss the issue of whether to seek further sanctions at a ministerial meeting next month.
He added that ministerial talks by six major powers on a new round of sanctions were likely to continue during the General Assembly session, scheduled from September 23 to October 1.
"The main thing to remember is the negotiating track is open ... There are contacts between the parties ... We need to focus very much on the negotiating opportunities which this may produce," Churkin said.
The Security Council has already ordered three rounds of sanctions against Iran. The United States says Iran is a weapons proliferation threat, while Iran insists that its nuclear research is for peaceful purposes.
Iran's latest letter to the international powers, delivered Tuesday, says only that "they are not prepared to move any further," according to another European diplomatic source in Brussels.
The letter said Tehran was ready to give a "clear response" to the international offer but demanded a "'clear response' to our questions and ambiguities."
Along with the threat of further sanctions, Washington has warned that the option of military action remains open.
And adding to signs of new diplomatic pressure, the International Atomic Energy Agency, the main UN nuclear watchdog, said its deputy director general, Olli Heinonen, would visit Tehran on Thursday for talks on the nuclear dispute.