Paris (AP) – France, Britain and Germany hardened their tone toward Iran, warning that Tehran risked triggering an international crisis and could face U.N. sanctions if it follows through with a threat to resume its nuclear program.

The toughened stance on Tuesday came a day after Iran said it would resume nuclear processing at its plant in Isfahan, beginning Wednesday, and follows the election of a new ultraconservative government in Tehran.

In a letter to Iran”s top nuclear negotiator, Hasan Rowhani, the three nations” foreign ministers and the EU foreign policy chief warned that restarting work would &#34terminate our dialogue&#34 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 U.N. Security Council, which in turn can impose sanctions.

The Tehran government rejected the European stance.

&#34The way is not to issue threats. Iran will not give in to threats,&#34 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 &#34de-escalating this dangerous situation.&#34

France warned that Iran would have to face the U.N. Security Council if it reopens its Isfahan Nuclear Conversion Facility and resumes uranium processing.

&#34This Iranian affair is very serious,&#34 said French Foreign Minister Philippe Douste-Blazy. &#34It could be the beginning of a major international crisis.&#34

Iran agreed on Monday to a two-day delay in reopening its Isfahan plant, after earlier vowing to immediately restart nuclear processing. Its announcement brought sharp responses from European officials who called on the Iranians to respect the terms of the Paris Agreement, which stipulated the nuclear program 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.

In Washington, State Department spokesman Tom Casey said the United States continues to support European efforts to resolve the issue through diplomacy. &#34It is critical to us that Iran maintains its suspension, that it maintains its adherence to the Paris agreement and that it not take any steps that would be in violation of that,&#34 Casey said. U.N. Secretary-General Kofi Annan urged &#34restraint and patience on the part of the Iranian authorities,&#34 his spokesman Stephane Dujarric said. Annan &#34very much believes they should wait for the latest proposals from the European Three before making any attempts to restart their nuclear activities.&#34

Iranian officials had signaled an intensifying impatience with the slow pace of negotiations with Europe. Bruno Tetrais of the Paris-based Foundation for Strategic Research said the Europeans were trying to publicly show Tehran, &#34We mean business&#34 and hit home a message that, &#34If you resume your enrichment activities, it”s a dramatic development that implies a new phase.&#34

Francois Gere of Paris” French Institute of Strategic Analysis said, &#34Today, the balance is shifting in the direction of a tougher tone.&#34

But Gere and other analysts said they were not optimistic for a breakthrough in the diplomatic standoff. &#34I do not see how the Iranians will come to agree that they do not have the right that other countries do,&#34 said Rime Allaf, from Britain”s Royal Institute of International Affairs. &#34In the diplomatic give-and-take both sides are staking out their positions.&#34

Iran suspended enrichment in November under international pressure and maintains its program is peaceful and only aimed at generating electricity as permitted under the Nuclear Nonproliferation Treaty. The EU and the United States fear the program is being used to develop nuclear weapons in violation of the treaty. The U.S.-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 &#34security guarantees&#34 that Iran won”t be invaded if it permanently halts uranium enrichment and related activities, European and Iranian officials confirmed.