US President Donald Trump has decided that five large countries will no longer be exempt from penalties if they continue to import Iranian oil.
The White House announced that sanctions waivers for China, India, Japan, South Korea and Turkey would not be renewed when they expired on 2 May.
The decision is intended to deny Iran its principal source of revenue.
President Donald Trump reinstated the sanctions last year after abandoning a landmark nuclear deal with Iran.
Under the 2015 accord with six world powers, Iran agreed to limit its sensitive nuclear activities and allow in international inspectors in return for sanctions relief.
The Trump administration hopes to compel Iran to negotiate a “new deal” that would cover not only its nuclear activities, but also its ballistic missile programme and what officials call its “malign behaviour” across the Middle East.
US officials have insisted they are not seeking “regime change”.
The sanctions have led to a sharp downturn in Iran’s economy, pushing the value of its currency to record lows, quadrupling its annual inflation rate, driving away foreign investors, and triggering protests.