Mohsen Fakhrizadeh, a man the Iranian government claimed was a prominent nuclear scientist, was assassinated in Iran this week.

The Islamic Republic accused Israel of being behind the killing and threatened to avenge the scientist’s death, while some analysts suggested the assassination was arranged by Iran officials.

Israel’s President Benjamin Netanyahu refused to comment on the assassination. Western intelligence groups said Fakhrizadeh was the mastermind behind a secret plan launched by the Iranian government to develop nuclear weapons.

Iran’s Foreign Minister Mohammad Javad Zarif said Iran was willing to engage in ongoing prisoner swaps, and suggested countries took part in a global exchange of Iranian prisoners with the regime. The comments were made following an exchange between detained British-Australian academic Dr. Kylie Moore-Gilbert and three Iranians imprisoned abroad.

Several dual nationals have been arrested in Iran in recent years on charges including espionage and “spreading corruption on Earth”. Human Rights campaigners have accused Iranian officials of using dual nationals as bargaining chips, which Tehran denies.

Iran’s Guardian Council approved legislation which halted UN inspections of its nuclear sites and allowed the regime to increase its uranium enrichment beyond the limits set out in the 2015 nuclear deal.

Iranian President Hassan Rouhani opposed the measures on Wednesday, saying they would harm any future diplomatic efforts to revive the nuclear deal and ease U.S. sanctions.

And a fresh round of penalties was issued by the United States, this time targeting the Shahid Meisami Group and its director, which the U.S. Treasury Department said had engaged in Iran’s chemical weapons research and had links to the Iranian Organization of Defensive Innovation and Research, which is currently blacklisted by Washington.