ISTANBUL — Iranian President Hassan Rouhani on Saturday blamed Israel for killing Mohsen Fakhrizadeh, a prominent Iranian nuclear scientist, saying it was aimed at causing turmoil before President-elect Joe Biden takes office and that Tehran would respond at the "right time."

Fakhrizadeh was killed in a daytime ambush east of Tehran on Friday, Iranian authorities said. Though once regarded as the architect of Iran’s disbanded effort to build a nuclear weapon nearly two decades ago, he played a far less prominent role in Iran’s current programs involving reactors and uranium enrichment.

But the attack stung Iran for apparent lapses in security and intelligence and threatened wider fallout in the region less than two months before Biden takes office and seeks a possible reset in relations with Iran.

Rouhani, however, suggested that Iran could calibrate its possible responses with an eye toward the end of President Trump’s “maximum pressure” campaign, which is supported by Israel.

“Their pressure era is coming to an end and the global conditions are changing,” he said. Officials in Israel have not commented.

A spokesperson for the European Union released a statement Saturday calling the attack “a criminal act” that “runs counter to the principle of respect for human rights the E.U. stands for.”

The statement continued, “In these uncertain times, it is more important than ever for all parties to remain calm and exercise maximum restraint in order to avoid escalation which cannot be in anyone’s interest.”

In a statement Saturday, Rouhani blamed the “usurper Zionist regime” — a reference to Israel — for the killing and said Fakhrizadeh’s death would not impede Iran’s scientific “achievements.” In a separate speech, Rouhani tied the killing to Trump’s upcoming departure from office.

Trump withdrew the United States from a nuclear pact that Iran struck with world powers five years ago. His administration has ratcheted up sanctions and other pressures on Tehran since abandoning the deal aimed at reining in Tehran’s nuclear program. Biden has pledged to work more closely with allies on Iran policies and to work to rejoin the nuclear agreement.

“This brutal assassination shows that our enemies are passing through anxious weeks, weeks that they feel their pressure era is coming to an end and the global conditions are changing,” Rouhani said. He added that Israel aimed “to cause commotion and turmoil.”

A senior U.S. official said that the United States had nothing to do with the scientist’s killing and believes Iran has been told that. The official, who spoke on the condition of anonymity to discuss a sensitive matter, said there was little doubt Israel was behind the attack.

“There is absolutely no information indicating that it was anyone other than the Israelis,” the official said, adding that the Trump administration has no desire to get drawn into a regional war by Israel.

An Iranian scientist was killed in an ambush near Tehran Nov. 27 that could elevate tension between Iran and its foes in the last weeks of the Trump presidency. (Reuters)

Germany — one of the world powers part of the nuclear pact — echoed the E.U.’s call for avoiding escalation.

“The killing of Mohsen Fakhrizadeh is once again worsening the situation in the region, at a time when we absolutely do not need such an escalation,” German Foreign Minister Heiko Maas told Germany’s Funke media group. He called on “all those involved to refrain from taking steps that could lead to a further escalation of the situation.”

The killing added to soaring tensions in the region amid fears that a confrontation between Iran and the United States or Israel could erupt before Biden takes office.

The front page of Iran’s hard-line Kayhan newspaper warned Israel to await an “eye for an eye.” Hamas and the Palestinian Islamic Jihad, two Iran-backed Palestinian militias, issued statements Friday condemning the assassination.

Lebanon’s Hezbollah, another Iranian ally, said Friday that “the response to this crime” was in Iran’s hands, Reuters reported.

Israel’s Channel 12 News reported Saturday that Israeli embassies around the world had been put on high alert.

The Pentagon announced Friday that the USS Nimitz aircraft carrier had been deployed back to the Middle East following maritime exercises in the Indian Ocean. While the timing of the announcement was unusual, the deployment had been initiated before Friday’s attack to support U.S. troops withdrawing from Iraq and Afghanistan.

A spokeswoman for the U.S. Navy’s Fifth Fleet, based in Bahrain, said “there were no specific threats that triggered the return of the Nimitz Carrier Strike Group,” Reuters reported.

Iran’s supreme leader, Ayatollah Ali Khamenei, warned in tweets Saturday that there would be “definitive punishment of the perpetrators,” while Iran would continue “following the scientific and technological attempts of the martyr in all segments of his activity,” in a possible reference to Fakhrizadeh’s nuclear work.

Fakhrizadeh, a physics professor who was believed to be about 60, was widely regarded as the driving force behind Iran’s nuclear program, including clandestine efforts to develop a nuclear bomb in the early 2000s.

Intelligence officials identified him as the head of the Amad Plan, the secret nuclear weapons research program that sought to develop as many as six nuclear bombs before Iranian leaders ordered a halt to the program in 2003.

Once a reclusive figure, Fakhrizadeh had more recently raised his profile, allowing himself to appear on official Iranian websites, including during events held by Iran’s supreme leader.

Iran has recently increased its stockpile of enriched uranium, insisting it is intended only to power its nuclear energy plants and a research reactor. Its adversaries have countered that it puts the nation closer to producing warhead-grade material.

Israel, which maintains its own undeclared nuclear weapons program, has beseeched the international community to take tougher actions against Iran’s nuclear program and Fakhrizadeh specifically. Prime Minister Benjamin Netanyahu highlighted the scientist’s photograph at a news conference in 2018 and advised the world to “remember his name.”

More recently, speculation has risen that Netanyahu has pushed the Trump administration to strike against Iran’s nuclear assets before Biden’s inauguration.

The Trump administration and the Biden team have yet to comment on the killing. On Friday, Trump retweeted veteran Israeli journalist Yossi Melman, who described the attack as a “major psychological and professional blow for Iran.”

Melman, author of “Spies Against Armageddon,” a history of Israeli clandestine operations, said in an interview that he would not be surprised to learn that the killing was an Israeli operation, but has gotten no such confirmation from within the government.

“There is total silence,” he said. “In this case, it is likely to stay that way.”

For Israel, Fakhrizadeh was “the number one target among the scientists,” he added.

While many saw the timing of the killing as an attempt to complicate a future Biden administration’s diplomacy with Iran, Melman said it was more likely driven by logistics, since such an ambush would have taken months to prepare.

The attack — which Iranian officials said involved a car bomb and gunmen who raked the scientist’s car — recalled the shadowy killings of at least four Iranian nuclear scientists from 2010 to 2012.

Accounts of Fakhrizadeh’s killing indicated his movements were being tracked and the attack — on a roadway east of Tehran near Mount Damavand, a peak that features in ancient folklore — was coordinated.

Sepah Cybery, a social media channel affiliated with Iran’s Islamic Revolutionary Guard Corps, said Saturday that it believed 12 attackers were involved, in addition to Fakhrizadeh’s four bodyguards, two of whom it reported were shot and are in serious condition.

Iran’s Fars news agency reported Friday that the car explosion sent debris flying 300 meters away and damaged an electricity tower. It also reported that one of the bodyguards was shot and killed while trying to protect Fakhrizadeh, though other outlets reported the security worker was injured.

In a message on Twitter on Friday, Sen. Chris Murphy (D-Conn.) wrote: “If the primary purpose of the killing of Mr. Fakhrizadeh was to make it harder to restart the Iran nuclear agreement, then this assassination does not make America, Israel or the world safer.”

Former CIA director John O. Brennan, a strong Trump critic, tweeted that the attack was “a criminal act & highly reckless.”

“It risks lethal retaliation & a new round of regional conflict,” he wrote. “Iranian leaders would be wise to wait for the return of responsible American leadership on the global stage & to resist the urge to respond against perceived culprits.”

Berger reported from Beirut and Hendrix from Jerusalem. Ellen Nakashima in Washington and Michael Birnbaum in Riga, Latvia, contributed to this report.