Iran's President Hassan Rouhani has accused arch-foe Israel of acting as a US "mercenary" and killing Mohsen Fakhrizadeh when assailants opened fire at him and his bodyguards near Tehran on Friday.
"Our people are wiser than to fall in the trap of the Zionist regime (Israel) ... Iran will surely respond to the martyrdom of our scientist at the proper time," he said.
Iran's clerical and military rulers have threatened revenge for Friday's killing of Mohsen Fakhrizadeh, which could further increase tensions in the Middle East region and beyond.
Biden, who defeated Donald Trump at the ballot box on November 3, has promised a return to diplomacy with Iran after four hawkish years under the departing US president.
The toughest measures - classified by health officials as "red" or level 3 - will be imposed in the capital Tehran and about 100 other towns and cities.
"The goal is not to shut down people's businesses, but we have to set limits," Raisi said.
The people of Iraq want good relations with the Islamic Republic of Iran based on the principle of non-interference in the internal affairs of both countries.
Khamenei also said Iran would not interfere in relations between Iraq and the United States, according to his official website.
Iran has been battling the spread of the coronavirus, with the total number of cases hitting 237,878 on Saturday and a further 148 deaths bringing the country's toll to 11,408.
"Government employees should not serve people who do not wear masks and employees who do not wear them should be considered absentees and sent home.