The toughened stance on Tuesday comes a day after Iran said it would resume nuclear processing at its plant in Isfahan, beginning on Wednesday, and follows the election of a new government in Tehran that the West views as ultra-conservative.
In a letter to Iran's top nuclear negotiator, Hasan Rowhani, the foreign ministers of the three European nations and the EU foreign policy chief warned that restarting work would "terminate our dialogue" and push the Europeans to seek a special session of the board of the International Atomic Energy Agency.
The agency's board can report countries to the UN Security Council, which in turn can impose sanctions.
No to threats
The Tehran government rejected the European stance.
"The way is not to issue threats. Iran will not give in to threats"
Hamid Reza Asefi,
Iranian Foreign Ministry
"The way is not to issue threats. Iran will not give in to threats," Iranian Foreign Ministry spokesman Hamid Reza Asefi said.
Earlier on Tuesday, France and Germany urged Iran to wait for a proposal from European negotiators that is expected this week.
German Chancellor Gerhard Schroeder said the European Union was prepared to offer extensive economic incentives to Iran, which he hoped could succeed in "de-escalating this dangerous situation".
France warned that Iran would have to face the UN Security Council if it reopened its Isfahan Nuclear Conversion Facility and resumed uranium processing.
"This Iranian affair is very serious," said French Foreign Minister Philippe Douste-Blazy. "It could be the beginning of a major international crisis."
Iran agreed on Monday to a two-day delay in reopening its Isfahan plant, after earlier vowing to immediately restart nuclear processing.
Negotiator Ali Agha Mohammadi
said nuclear work would resume
That announcement brought sharp responses from European officials who called on the Iranians to respect the terms of the Paris agreement, which stipulated the nuclear programme must remain frozen until negotiations were finished.
France, Germany and Britain, negotiating on behalf of the European Union, have urged Iran not to take unilateral action, saying they are only days from delivering a package of incentives addressing security and political, economic and nuclear issues.
Iranian officials had signalled an intensifying impatience with the slow pace of negotiations with Europe.
In Washington, State Department spokesman Tom Casey said the United States continued to support European efforts to resolve the issue through diplomacy.
"It is critical to us that Iran maintain its suspension, that it maintain its adherence to the Paris agreement and that it not take any steps that would be in violation of that," Casey said,
UN Secretary-General Kofi Annan urged "restraint and patience on the part of the Iranian authorities", his spokesman Stephane Dujarric said.
Annan "very much believes they should wait for the latest proposals from the European Three before making any attempts to restart their nuclear activities".
Bruno Tetrais of the Paris-based Foundation for Strategic Research said the Europeans were trying to publicly show Tehran "we mean business" and hit home a message that:
"If you resume your enrichment activities, it's a dramatic development that implies a new phase."
Tehran plans to resume nuclear
work at Isfahan on Wednesday
Francois Gere of the French Institute of Strategic Analysis said: "Today, the balance is shifting in the direction of a tougher tone."
Gere and other analysts said they were not optimistic for a breakthrough in the diplomatic standoff.
"I do not see how the Iranians will come to agree they do not have the right that other countries do," said Rime Allaf, from Britain's Royal Institute of International Affairs.
"In the diplomatic give-and-take, both sides are staking out their positions."
Iran suspended enrichment in November under international pressure but maintains its programme is peaceful and only aimed at generating electricity as permitted under the Nuclear Non-proliferation Treaty.
The EU and the US fear the programme is being used to develop nuclear weapons in violation of the treaty.
The US-backed European proposal is expected to be presented to Iran by Sunday, according to the French and German foreign ministries.
It includes nuclear fuel, technology, other aid and "security guarantees" that Iran will not be invaded if it permanently halts uranium enrichment and related activities, European and Iranian officials said.