The United States will continue to impose sanctions on whoever purchases Iran's oil or conducts business with Iran's Revolutionary Guards and no oil waivers will be re-issued, a U.S. official told Reuters on Sunday.

"We will continue to put pressure on Iran and as President (Trump) said there will be no waivers of any kind for Iran's oil," said Sigal Mandelker, U.S. Treasury Under Secretary for Terrorism and Financial Intelligence.

Mandelker added that Iranian oil sales have taken a "serious nose dive" because of U.S. pressure.

Iran's crude oil exports were slashed by more than 80% due to the re-imposed sanctions by the United States after President Donald Trump exited last year Iran's 2015 nuclear deal with world powers.

Since ditching the nuclear deal, calling it skewed to Iran's advantage, Trump has reimposed sanctions to strangle its vital oil trade and force Tehran to accept stricter limits on its nuclear activity, curb its ballistic missile programme and end its support for proxy forces around the Middle East.

In retaliation, Iran has been reducing its commitments under the deal since May, pressuring European countries to the pact to protect Tehran's interests and its economy.

France has proposed offering Iran about $15 billion in credit lines until year-end if Tehran comes fully back into compliance with its 2015 nuclear deal, a move that hinges on Washington not blocking it, Western and Iranian sources said.

In addition to saving the deal, Tehran wants to restart selling its oil.

Two Iranian officials and one diplomat told Reuters on Aug. 25 that Iran wants to export a minimum of 700,000 barrels per day of its oil and ideally up to 1.5 million bpd if the West wants to negotiate with Tehran to save the nuclear deal.