A coffin with an image of Iranian nuclear scientist Mohsen Fakhrizadeh can be seen among the servants of the holy shrine of Imam Reza in Mashhad, Iran, November 29, 2020. Photo: Massoud Nozari/WANA (West Asia News Agency) via REUTERS.

Iran will give a “calculated and decisive” response to the killing of its top nuclear scientist, said a top adviser to Iran’s supreme leader.

“Undoubtedly, Iran will give a calculated and decisive answer to the criminals who took Martyr Mohsen Fakhrizadeh from the Iranian nation,” Kamal Kharrazi, who is also head of Iran’s Strategic Council on Foreign Relations, said in a statement.

Fakhrizadeh, long suspected by Western and Israeli governments of masterminding a secret nuclear weapons program, was ambushed on a highway near Tehran on Friday and gunned down in his car.

Iran’s clerical and military rulers have blamed the Islamic Republic’s longtime enemy, Israel, for the killing. Iran has in the past accused Israel of killing several Iranian nuclear scientists since 2010.

Prime Minister Benjamin Netanyahu’s office has declined to comment on the killing. An Israeli cabinet minister, Tzachi Hanegbi, said on Saturday he did not know who carried it out.

Iranian hardline media called on Sunday for a tough revenge. However, Iran’s rulers are aware of daunting military and political difficulties of attacking Israel. Such an attack would also complicate any effort by US President-elect Joe Biden to revive detente with Tehran after he takes office on Jan. 20.

Tensions have been high between Tehran and Washington since 2018, when President Donald Trump exited Iran’s 2015 nuclear deal with six major powers and reimposed sanctions that have hit Iran’s economy hard. In retaliation, Tehran has gradually breached the deal’s curbs on its nuclear program.

Biden has said he will return the United States to the deal if Iran resumes compliance. Iran has always denied pursuing nuclear weapons.