Iran’s leaders threatened on Saturday to retaliate over the assassination of the country’s top nuclear scientist, blaming Israel and pledging to continue the work of the man who American and Israeli officials believe was the architect of the country’s secretive nuclear weapons program.
But Germany, a key U.S. ally in Europe, urged all sides not to allow the last weeks of the Trump administration to obliterate hopes for fresh negotiations over Iran’s nuclear program. The comments followed the assassination on Friday of the scientist, Mohsen Fakhrizadeh, outside the Iranian capital, Tehran.
“A few weeks before the new U.S. administration takes office, it is important to preserve the scope for talks with Iran so that the dispute over Iran’s nuclear program can be resolved through negotiations,” a spokesman for Germany’s Foreign Ministry said in a statement. “We therefore urge all parties to refrain from any steps that could lead to a further escalation of the situation.”
President Trump’s 2018 withdrawal from the nuclear accord between Iran and six world powers unraveled the signature foreign policy achievement of his predecessor, Barack Obama, and distanced the United States from Western allies, including Germany, which tried to keep the agreement intact.
Now, President-elect Joseph R. Biden Jr. wants to reactivate the 2015 accord, which curtailed Iran’s nuclear activities, but the killing could complicate that aim.
Ayatollah Ali Khamenei, Iran’s supreme leader, said on Saturday that Iranian officials must commit themselves to “pursuing this crime and punishing its perpetrators and those who commanded it.”
And Hassan Rouhani, Iran’s president, blamed Israel for the assassination and said in a televised cabinet meeting that Iran would respond “in due course.”
“Iran’s enemies should know that the people of Iran and officials are braver than to leave this criminal act unanswered,” he said on Saturday. “In due time, they will answer for this crime.”
In response, Israel on Saturday put its embassies around the world on high alert, Israeli N12 News reported. The country’s Foreign Ministry said it would not comment on embassy security matters.
For decades, Mr. Fakhrizadeh was the guiding figure behind what American and Israeli officials have described as a covert campaign by Iran to design an atomic warhead. He was shot in what the Iranian news media said was a roadside ambush as he and his bodyguards traveled outside Tehran.
One American official and two other intelligence officials said that Israel was behind the attack on the scientist. It was unclear how much the United States may have known about the operation in advance, but the two nations are the closest of allies and have long shared intelligence regarding Iran, which Israel considers its most potent threat.
Since the nuclear deal collapsed, Iran, which maintains that its nuclear ambitions are for peaceful purposes rather than weapons, has again begun increasing its nuclear capabilities.
John Brennan, who was C.I.A. director under Mr. Obama, said on Twitter on Friday that the killing was a “criminal act & highly reckless” and that it risked “lethal retaliation & a new round of regional conflict.”
The remarks on Saturday by Iran’s leaders fueled the concerns of a new escalation of retribution.
“Once again, the evil hands of global arrogance and the Zionist mercenaries were stained with the blood of an Iranian son,” Mr. Rouhani said, echoing phrases that Iranian officials use to refer to Israel.
Analysts have said that the killing would set back Iran’s nuclear program. But Mr. Rouhani tried to dispel that idea, saying that it would “not slow down our achievements” and reinforcing promises from several Iranian leaders that they would not relent in their nuclear ambitions.
In a letter to the United Nations secretary general, António Guterres, and the Security Council on Friday, the Iranians said there were “serious indications” that Israel had carried out the killing, and that they reserved the right to retaliate.
