“Importing vaccines made in the US or the UK is prohibited,” Ayatollah Ali Khamenei said in a tweet, accompanied by the hashtag #CoronaVaccine.
“It’s not unlikely they would want to contaminate other nations,” he added.
The Islamic republic has reported more than 1.2 million cases of the novel coronavirus, which have caused over 56,000 deaths.
Iran has accused arch-enemy the US of hampering its access to vaccines through a tough sanctions regime.
While food and medicine are technically exempt from the measures, international banks tend to refuse transactions involving Iran.
Khamenei also tweeted that “given our experience with France’s HIV-tainted blood supplies, French vaccines aren’t trustworthy either.”
That was a reference to a scandal in the 1980s in which blood infected with HIV was distributed in France, and later abroad, even after the government became aware of the problem.
Hundreds of people in Iran were among those infected.
France’s then-prime minister Laurent Fabius was charged with manslaughter, but acquitted in 1999, while his health minister was convicted but never punished.
Iran last month launched clinical trials of a vaccine developed in the Islamic republic, the Middle Eastern country hardest hit by the pandemic.