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 US 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, US treasury under secretary for Terrorism and Financial Intelligence.

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

Iran's crude oil exports were slashed by more than 80 percent 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, Germany and Britain have tried to launch a barter trade mechanism with Iran protecting it from US sanctions but have struggled to get it off the ground, and Tehran on Wednesday set a 60-day deadline for effective European action.

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

Two Iranian officials and one diplomat told Reuters on August 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.