"I determine … that there is a sufficient supply of petroleum and petroleum products from countries other than Iran to permit a significant reduction in the volume of petroleum and petroleum products purchased from Iran by or through foreign financial institutions," Trump said in the memorandum on Wednesday.
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Trump added that he will continue to monitor the situation with respect to the oil embargo on Iran closely.
The United States re-imposed sanctions on Iran after withdrawing from the 2015 nuclear agreement with that country in May, a move strongly condemned by Tehran.
The first round of sanctions was re-introduced in August, but tougher restrictions mainly targeting the Iranian oil industry are expected to go in effect on November 5 as the United States seeks to squeeze Tehran out of the global energy market.