Iran leader bans import of US and UK-made Covid vaccines
"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.
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.