Iran - US President Donald Trump has decided to end exemptions from sanctions for countries still buying oil from Iran.
The White House said waivers for China, India, Japan, South Korea and Turkey would expire in May, after which they could face US sanctions themselves.
This decision is intended to bring Iran's oil exports to zero, denying the government its main source of revenue.
Iran insisted the sanctions were illegal and that it had attached "no value or credibility" to the waivers.
Mr Trump reinstated the sanctions last year after abandoning a landmark 2015 nuclear deal between Iran and six world powers.
Under the accord, Iran agreed to limit its sensitive nuclear activities and allow in international inspectors in return for sanctions relief.
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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.
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.