Speaking at an informal meeting of EU foreign ministers, EU foreign policy chief Javier Solana said the bloc was watching to see if Iran would comply with a deadline this week set by the International Atomic Energy Agency (IAEA).
He noted that the UN nuclear watchdog’s board of governors is set to meet on September 19 in Vienna, after a September 3 deadline for Iran to stop work on making nuclear reactor fuel that could also be used to make weapons.
"Then we’ll see," Solana told reporters, adding that the EU would like to avoid escalating the situation "but (it would) be ready to go to the (UN) Security Council if necessary, yes."
British Foreign Secretary Jack Straw, chairing the EU meeting as part of Britain’s turn at the EU presidency, was more measured in his remarks, saying the bloc was proceeding "one step at a time".
"The EU very much hopes Iran will reconsider its position," he said. "The key to resolving this issue is for Iran to take the confidence-building steps requested of it."
The United States suspects that the Islamic republic is trying to develop nuclear weapons, and has long threatened to refer the issue to the UN Security Council.
But the European bloc resisted that call, and launched negotiations with Tehran — offering trade and other benefits in exchange for pledges on its nuclear plans — after striking an accord with it in Paris last November.
Those talks — spearheaded by the so-called EU-3 of Britain, France and Germany, along with Solana — however broke down last month after Iran ignored calls not to resume sensitive nuclear activities.
"We are very very disappointed. We have been offering a lot to the Iranians. We again call on the Iranians that they come back to the negotiating table," said EU external relations commissioner Benita Ferrero-Waldner.
Asked if the Iran dossier may now have to be referred to New York, she said: "For the time being nothing can be ruled out."
"We are at a very delicate moment. Of course the Iranians should know that the door is still open… Nobody wants to go to the Security Council unless it became unavoidable," she said.
French Foreign Minister Philippe Douste-Blazy said: "We are not shutting the door… The Iranians must and can go back to the Paris agreement of November 2004 and, thus, negotiation."
Analysts say that the threat to refer Tehran to the UN could in any case be an empty one, since any sanctions could be vetoed by permanent seat holders Russia or China, both of which have close ties with the Islamic state.
But meanwhile Iran has threatened retaliatory measures if it is taken to the Security Council, with analysts saying this could involve withholding oil from the world market or simply going ahead with enrichment.
Other EU sources said that the warning from Solana about refering Tehran to the UN was not a threat.
"Today is not a day for ultimatums," said one, while another added: "The main aim is to persuade Iran to come back to the negotiating table."
"Nobody now excludes the possibility that we will be obliged to go to the Security Council," said another. "There is still a window of opportunity, but its getting smaller."