BEIRUT — Amid vows to avenge the killing of scientist Mohsen Fakhrizadeh, Iran's leadership promised Monday to push ahead with its nuclear program while casting doubt on the future of negotiations with the West.

Fakhrizadeh’s assassination — and the pressure now on Iran’s leadership to retaliate after an embarrassing security failure — could undermine President-elect Joe Biden’s pledge to return to a negotiated nuclear deal with Iran and world powers. President Trump left the pact in 2018 while ratcheting up economic sanctions and pressure on Iran.

Iran has blamed Israel for Fakhrizadeh’s death, which has elevated uncertainty in the region after Iranian leaders pledged a “definitive punishment” and to respond “at the right time.”

“Some say through dialogue and negotiations actions can be taken in order to put an end to such hostility,” Iran’s supreme leader, Ayatollah Ali Khamenei, said in a statement read at Fakhrizadeh’s funeral by a representative, according to a translation by Iranian state television. “This is not possible, because our enemies oppose the nature of the Islamic Republic establishment. . . . They will never put an end to their hostilities toward us.”

Fakhrizadeh was a driving force behind an Iranian effort to build a nuclear weapon, an effort that U.S. intelligence says was abandoned nearly two decades ago. His role in Iran’s current nuclear power programs involving reactors and uranium enrichment was less direct.

Israel has not officially commented on the Friday attack in keeping with its policy of not speaking out on security matters. Israeli Intelligence Minister Eli Cohen told Israeli radio on Monday that he did not know who was behind the ambush.

The European Union, the United Nations and Germany, among others, have condemned the assassination and called for restraint. British Foreign Secretary Dominic Raab said Sunday that Britain was “concerned” about the killing.

“We’re still waiting to see the full facts, to address the full facts of what’s happened in Iran, but I would say that we stick to the rule of international humanitarian law, which is very clear against targeting civilians,” Raab said.

Iranian media and officials have reported conflicting accounts of the ambush midday Friday of Fakhrizadeh and his security team. Reports, citing witnesses, initially said that a car bomb went off and that a firefight broke out between his bodyguards and up to 12 assassins who escaped.

The Fars News Agency, which is close to Iran’s Revolutionary Guard Corps, reported Sunday that no attackers were present and that the perpetrators instead used a remote-controlled machine gun. Iran’s English-language Press TV added Monday that a weapon with an Israeli logo was located at the scene. Ali Shamkhani, secretary of the Supreme National Security Council of Iran, told state TV that Iranian opposition groups in exile were also involved, Reuters reported.

Fakhrizadeh’s killing recalled the shadowy assassinations of Iranian nuclear scientists a decade ago, which Iran also blamed on Israel.

The scientist was buried in Tehran in a ceremony attended by Iranian officials and family members but closed to the public because of coronavirus-related restrictions. Over the weekend, his body was flown to the city of Mashhad, where it was taken in a procession to the revered Imam Reza shrine, and then to the holy city of Qom.

Iran’s government provided Fakhrizadeh’s funeral the same high-level honors that it did for Iranian commander Qasem Soleimani, whom the United States killed in Iraq in January. Hundreds of thousands attended Soleimani’s funeral processions, and dozens died in resulting stampedes.

Soon after Fakhrizadeh’s death, artwork began circulating online of the scientist alongside Soleimani and Iraqi militia leader Abu Mahdi al-Muhandis — three high-level figures assassinated this year.

In his statement Monday, Khamenei promised to “find the perpetrators of the plot and follow up on [Fakhrizadeh’s] research efforts.”

Fakhrizadeh’s death has emboldened critics of President Hassan Rouhani, a centrist reformer who has championed dialogue and Iran’s 2015 nuclear deal with the United States and five other world powers.

Iran’s parliament met Sunday for a closed-door intelligence briefing about Fakhrizadeh’s assassination. Abolfazl Amouyee, a spokesman for the Iranian parliament’s National Security and Foreign Policy Commission, accused the international inspectors tasked with monitoring the country’s nuclear power program of possible espionage and called for their access to be limited. “We are sensitive toward the espionage of the IAEA inspectors,” Amouyee said, according to Fars, referring to the International Atomic Energy Agency.

Karim Sadjadpour, a senior fellow at the Washington-based Carnegie Endowment for International Peace, said in a tweet Sunday that Iran’s leadership now faces a dilemma over how to respond to Fakhrizadeh’s killing without compromising other demands, such as U.S. sanctions relief. “To restore its economy Iran needs a full or partial return to the nuclear deal. To restore deterrence/pride it will want to avenge Fakhrizadeh’s death. Doing the latter without sabotaging the former is most difficult. That’s one reason why Fakhrizadeh was killed.”