UNITED NATIONS – European officials signaled yesterday for the first time that they may cease diplomatic relations with Iran after Tehran indicated it might resume nuclear activity that could bring it closer to producing an atomic bomb. The European countries could join America in threatening sanctions.

The foreign ministers of Britain, Germany, and France wrote a letter to Tehran warning that resumption of nuclear activity would end two years’ worth of negotiations. Prime Minister Blair said yesterday that Europeans could soon send Iran to the Security Council to face possible sanctions, a move America has urged in the past.

“We certainly will support referral to the U.N. Security Council if Iran breaches its undertaking and obligations,” Mr. Blair said in London during his first press conference since winning re-election last week. Germany’s foreign minister, Joschka Fischer, hinted last week at taking such a tack, but refused to repeat the threat following consultations with his European partners.

Israel’s foreign minister, Silvan Shalom, yesterday dismissed the possibility of diplomacy altogether. In Mr. Shalom’s country, Iran’s pursuit of nuclear weapon capabilities ranks as a key concern.

“The stick-and-carrot approach to Iran is not working,” Mr. Shalom told reporters at a briefing on Israel’s Independence Day. “The possibility that Iran could threaten the world with one push of a button is a nightmare,” he said. He added that the matter should be turned to the Security Council immediately.

Germany, Britain, and France toughened their joint diplomatic stance against Iran upon learning that the country intended to resume enrichment despite past promises to refrain from doing so. An Iranian negotiator arrived in Vienna Wednesday with a sealed letter that contained word of Iran’s intention to renew uranium-reprocessing work at Isfahan. Europeans insist that Iran has promised it would refrain from enrichment – the first step toward weapons capability.

In Tehran, the top nuclear negotiator, Hasan Rowhani, said yesterday the negotiations with the three European powers must change fundamentally. “Continuation of negotiations in their present format is not possible for us,” Mr. Rowhani told Iranian television. “The basic point that the Islamic Republic of Iran will resume part of its nuclear activities in the near future is definite.”

America has been a longtime skeptic of European attempts to dissuade Iran from developing nuclear weapons through negotiations alone. Arguing that Iran’s mullahs are less than truthful, the Bush administration has pushed for a confrontational approach that would include threatening Iran with punishment.

Recently, however, Washington has changed course by several degrees, officially supporting the three European powers in their diplomacy. Meantime, the White House privately urged them to show results. The White House “continues to support the efforts by the Europeans to resolve this matter and to make sure there’s an objective guarantee in place,” a spokesman, Scott McClellan, said yesterday, adding, “Iran needs to abide by its international obligations.”

The European foreign ministers’ harshly worded letter to Mr. Rowhani, first reported by the Washington Post yesterday, stated that if Iran resumes nuclear work in coming days, it “would bring the negotiating process to an end.” The letter reads, “The consequences could only be negative for Iran.”

According to diplomats in Vienna and officials at the International Atomic Energy Agency, Iranians intended to notify the agency that it might soon break the seals the IAEA has put on nuclear material. Under previous agreements, that material was never expected to be used.

If Iran acts to release the material, one diplomat told The New York Sun on condition of anonymity that the head of the IAEA, Mohammed el-Baradei, would immediately call an emergency meeting of the agency’s board of directors next week. That step would likely lead to a referral to the Security Council.

Vice President Cheney has refused to discount the possibility that the Israeli military might attack Iranian facilities. America has supplied Israel with airborne bombs, known as “bunker busters,” which could hit deeply dug targets such as the Iranian nuclear facilities.

While saying they support the international diplomacy on Iran, Israeli officials left open the military option. “Iran advocates terror and Islamic extremism, and is ruled by an unconventional regime that might develop unconventional arms,” the army chief of staff, Lieutenant Colonel Moshe Ya’alon, told Israel Defense Force Radio yesterday. “Israel should defend itself against any threat, and it will always know how to do so.”

Israeli intelligence analysts consider the Iranian nuclear threat to be more acute than do their American counterparts. Israel argues Iran could reach a “point of no return” in months, while America believes it’s a matter of years. According to the Israeli newspaper Ha’aretz, while both sides rely on the same raw intelligence data, the difference stems from the definition of the threat.

Israel believes Iran would be close to achieving nuclear weapons capability if it could enrich uranium independently. This might happen by the end of this year, the Israelis say, and after three more years, Iran could create enough fissile material for a bomb. The Americans see the point of no return as the point at which Iran could amass 55 pounds of highly enriched uranium, or 15 pounds of plutonium – the minimum fissile material needed for a bomb.