In a statement last Wednesday, Treasury Secretary Steven Mnuchin said Zarif "implements the reckless agenda of Iran's Supreme Leader, and is the regime's primary spokesperson around the world".
Zarif's press conference came a day after Iran announced its forces had seized a foreign ship in the Persian Gulf suspected of carrying smuggled fuel.
Carlos Martin added: "From our side, we will continue to work with Mr Zarif as Iran's most senior diplomat and in view of the importance of maintaining diplomatic channels".
Tensions soared after the Trump administration stepped up a United States campaign of "maximum pressure" against Iran, with drones downed and tankers mysteriously attacked in Gulf waters. Iraq has denied the accusation, saying it "does not export diesel to the global market".
"What we've been doing is working to ensure that there is a coalition in place, which can ensure safe passage through what is a very important strait", Prime Minister Boris Johnson's spokesman, James Slack, told reporters on Monday. "The ship was seized in Iranian territorial waters and had been transporting diesel fuel", it added.
On July 18, the paramilitary force seized a United Arab Emirates-based oil tanker, the Panamanian-flagged MT Riah, for allegedly smuggling some 1 million liters (264,000 gallons) of fuel from Iranian smugglers to foreign customers.
Mnuchin reportedly argued that without renewing the waivers, "the United States would have to sanction Russian, Chinese and European firms that are involved in projects inside Iran that were established as part of the 2015 nuclear deal".
Last month, Iran seized a British-flagged tanker in the strategic Strait of Hormuz, in a retaliatory action for the seizure of an Iranian vessel by Gibraltar - a British overseas territory. US President Donald Trump pulled the US out of the deal last May and reimposed sanctions on Iran, endangering the implementation of the agreement also endorsed by the UN Security Council.
"All countries which have interests in the region are by no means willing to see a new crisis in the Middle East", he said, quoted by Mehr news agency. Iran denied any involvement.
Speaking at a press conference on Monday in Tehran, Zarif said that during his visit to the United Nations last month he met with several U.S. senators and congressmen.
Britain now has deployed a destroyer HMS Duncan and a frigate HMS Montrose to the Gulf to accompany UK-flagged vessels through the strait.
In response to such incidents, the U.S. has been searching for to kind a coalition whose mission it says is to ensure freedom of navigation within the Gulf. Iranian media reported on Sunday that Iranian and Qatari coast guards will meet to "improve maritime and border cooperation".
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