The US is prepared to work with nations that are reducing their oil imports from Iran on a "case-by-case basis", but will not grant waivers to countries like India and Turkey as it could substantially reduce pressure on sanctions-hit Tehran, according to a senior Trump administration official.

Iran is India's third-largest oil supplier behind Iraq and Saudi Arabia. Iran supplied 18.4 million tonnes of crude oil during April 2017 and January 2018 (first 10 months of 2017-18 fiscal).

Last month President Donald Trump withdrew the United States from the 2015 landmark Iran nuclear deal, re-imposing US sanctions that had been suspended in return for curbs on Tehran's nuclear programme.

At the time, the Trump administration gave foreign companies either 90 or 180 days to wind down their business with Iranian counterparts, depending on the type of commercial activity.

Now, Washington is stepping up pressure on all countries, including India and China, to completely stop buying oil from Iran by November 4.

"We are not looking to grant licenses or waivers, because doing so would substantially reduce pressure on Iran, and this is a campaign of imposing pressure," Brian Hook, Director of Policy Planning at the State Department told reporters at a news conference yesterday.

"And so, we are not looking to grant licenses or waivers broadly on the reimposition of sanctions, because we believe pressure is critical to achieve our national security objectives," he said.

Hook said that the first part of US sanctions against Iran will snap back on August 6.

"These sanctions will include targeting Iran's automotive sector, trade in gold, and other key metals," he said.

"The remaining sanctions will snap back on November 4. These sanctions will include targeting Iran's energy sector and petroleum-based transactions, and transactions with the Central Bank of Iran," Hook said.

"We are prepared to work with countries that are reducing their imports on a case-by-case basis, but as with our other sanctions, we are not looking to grant wavers or licenses," Hook said when asked about India and Turkey which import Iranian oil.

After Trump announced his withdrawal from the Iranian nuclear agreement, Hook said American officials have been visiting several world capitals to convey president Trump's  message of cooperation and coordination on the Iran issue.

"Many countries around the world share our interests in countering terrorism, halting the proliferation of missiles and promoting peace and stability in the Middle East. We want to work with these countries to build a strong global effort,” he said.

"Our focus is on getting as many countries importing Iranian crude down to zero as soon as possible. We are also working with oil market participants, including producers and consumers, to ensure market stability.

"Banking sanctions will also snap back on November 4th, and we will be aggressively enforcing these provisions to lock up Iran's assets overseas and deny the Iranian regime access to its hard currency," Hook said.