President Hassan Rouhani said Iran will respond to the killing of its top nuclear scientist “when the time is right,” accusing Israel of an “act of terrorism” in a significant escalation of tensions in the Persian Gulf.

Mohsen Fakhrizadeh, a senior nuclear scientist working for the Ministry of Defense, was assassinated Friday in a shootout and car bombing on the outskirts of Tehran. Promising “severe revenge,” officials and military commanders also pointed the finger at the U.S., an ally of Israel, potentially complicating President-elect Joe Biden’s bid to revive the beleaguered Iranian nuclear deal.

The chances of salvaging that international accord, which restricts Iran’s nuclear program and prevents it from building a weapon, were already in doubt as Donald Trump seeks to use the final weeks of his presidency to cement America’s withdrawal. The European Union, which wants to keep the pact alive, described Friday’s attack as a “criminal act” that “runs counter to the principle of respect for human rights,” while urging “maximum restraint.”

Protesters set fire to representations of the U.S. and Israeli flags during a gathering in front of the Iranian Foreign Ministry on Saturday in Tehran. Supreme Leader Ayatollah Ali Khamenei is calling for “definitive punishment” of those behind killing of Mohsen Fakhrizadeh, the scientist linked to Tehran’s disbanded military nuclear program. (AP Photo/Vahid Salemi)

Commenting Saturday on his official website, Rouhani referred to the “mercenary Israeli regime.” Supreme Leader Ayatollah Ali Khamenei vowed to pursue Fakhrizadeh’s killers and continue his scientific work.

Rouhani’s remarks on the timing of a possible response echo what Iran said after the U.S. assassinated General Qasem Soleimani in January – an act that almost triggered a full-scale war between the two foes. Iran retaliated five days later by firing rockets at an American base in Iraq.

No serious casualties were reported in that strike. But in the aftermath, the Islamic Revolutionary Guard Corps shot down a Ukrainian passenger jet it mistook for a missile as it took off from Tehran, killing 176 people, mostly Iranian nationals.

Qatar, which has had a tense relationship with close U.S. allies Saudi Arabia, Egypt, Bahrain and the U.A.E. since the quartet subjected the peninsula state to a blockade in 2017, strongly condemned the attack, according to the state-run Qatar News Agency, citing a phone call between Foreign Minister Mohammed bin Abdulrahman Al Thani and his Iranian counterpart Mohammad Javad Zarif.

Zarif appeared to take a dig at the United Nations and other governments that have called for restraint, saying on Twitter that they refused to take a stand against terrorism.

Axios reported this week that Israel’s government had instructed the military to prepare for a possible U.S. strike against Iran during the remainder of Trump’s term, though it said the order wasn’t based on intelligence or an assessment that the U.S. would order an attack.

The office of Israeli Prime Minister Benjamin Netanyahu declined to comment, as did officials at the Pentagon. The Central Intelligence Agency didn’t immediately respond to questions Friday about whether the U.S. had knowledge of plans to carry out an assassination.

Trump retweeted a New York Times report on Fakhrizadeh’s killing without comment.

A senior nuclear official linked the assassination to an explosion in July that caused significant damage to Iran’s largest uranium enrichment facility in Natanz. Behrouz Kamalvandi, spokesman for the Atomic Energy Organization of Iran, said Israel was behind both the fire at that plant and Fakhrizadeh’s death, the semiofficial Iranian Students’ News Agency reported.

Fakhrizadeh was the head of research and innovation at Iran’s Ministry of Defense and widely seen as having a major role in the country’s nuclear program. He was near the Rudehen campus of the Islamic Azad University, about 60 kilometers (37 miles) east of central Tehran, when he was killed, the semiofficial Tasnim news service reported.

He had been singled out by Netanyahu in an April 2018 presentation on Iran’s nuclear program, claiming the scientist headed a secret project to develop nuclear arms.

The International Atomic Energy Agency, a United Nations watchdog, has said “Project Amad” was mothballed in 2003. Iran dismissed Netanyahu’s presentation as “lies and warmongering.”

On Monday, Israeli media reported that Netanyahu had secretly flown to Saudi Arabia to meet Crown Prince Mohammed bin Salman and U.S. Secretary of State Michael Pompeo the previous day.

The three countries, along with the United Arab Emirates, have built an anti-Iran axis in the Middle East over the past three years, based mainly on the U.S.’s so-called maximum-pressure strategy, which involved Trump walking away from the 2015 nuclear deal and imposing wide-ranging sanctions on Iran’s economy.

Iranian state media has been dominated by coverage of Fakhrizadeh’s death since Friday evening. News reports have emphasized his importance in defending the nuclear program, as well as recent projects related to the coronavirus, including the production of diagnostic kits and efforts to produce a vaccine.