Donald Trump last month refused to give waivers to countries like India from buying oil from Iran.

Any efforts to import Iranian crude oil beyond the accepted levels negotiated from November to May will be sanctioned, the US has warned, amidst reports that India and China were seeking ways to buy oil from Tehran.

US President Donald Trump last month refused to give waivers to countries like India from buying oil from Iran, in an attempt to reduce Iran's oil exports to zero.

The US on Tuesday reiterated its position after the media reports from New Delhi, quoting unnamed government officials, said that India was looking at ways to resume oil imports from Iran despite the US sanctions.

Last week, India''s Ambassador to the US Harsh Vardhan Shringla said India had stopped buying oil from Iran after May 2 when the US ended its waivers that allowed the top buyers of Iranian oil, including India, to continue their imports for six months.

"There will be no more oil waivers granted and the only oil that would have been permitted would have been under the cap that we negotiated. That cap was negotiated, it was to run for a period from November of 2018 until May of 2019," Brian Hook, Special Representative for Iran and Senior Policy Advisor to the Secretary of State, told reporters during a conference call on Thursday.

He said once the countries have reached the cap of what was negotiated, that would be the limit of the oil that US would permit to move through and would not be sanctioned.

"We will sanction any efforts to import Iranian crude oil beyond the limits that were negotiated in the period that ran from November through May," Hook said.

Responding to the question that India and China were importing a little bit of Iranian oil, he said: "The countries that you mentioned I think every country is aware of it. We now have close to 30 countries that used to import Iranian crude oil that are now at zero. This accounts for 40 per cent of the regime's revenue".

Iran earlier used to supply 10 per cent of India''s oil needs.

After coming to power, President Trump withdrew from the Iranian nuclear deal last year and has imposed stringent sanctions against what he describes as the "authoritarian" Iranian regime.

"If we want to get serious about denying Iran the money it needs to destabilise the Middle East, we have to enforce oil sanctions. We think all countries in the world share a desire for a more peaceful and stable Middle East. For as long as Iran is able to conduct its foreign policy with impunity fuelled by oil revenue, it is going to be unstable," Hook said.

That has been the message of the Trump Administration to all these countries, he said.

"Nations know that if there are efforts to import Iranian crude oil beyond the accepted levels that were negotiated back from November through May, they will be sanctioned," he asserted.

State Department Spokesperson Morgan Ortagus reiterated that it was the policy of the Trump administration to take Iranian oil export to zero and there will be no extensions on these waivers.

"That remains our policy," she said. Ortagus said the US policy on Iran was working.

"We see the compliance with US sanctions regarding Iran writ large to be one of the most successful things that this administration and that this State Department has done," she said.

Later, a Senior State Department official told a group of reporters that India''s developing of the Chabahar port in Iran was exempted from the Iranian sanctions.

"That exemption continues. It''s not being reviewed. The exemption allows for the development of the port, the development of the rail link to Afghanistan, the provision of crude oil and gas to Afghanistan and the provision of humanitarian supplies as well. That continues," the official said.

The official said the Trump administration welcomed India''s efforts to further develop trade with Afghanistan.

India continued to play a leading role and is providing humanitarian assistance, including wheat, which has gone through the Chabahar port.

The US was having an ongoing conversation with India on Iran, the official said, adding that America welcomes the fact that India has reduced its crude oil imports from Iran.

"I don''t see frankly a difference in concerns over the prospect of a nuclear Iran. I don''t believe it''s in India''s interest and Indian officials are forthright in saying that they do not want to see a nuclear Iran," the official said.

"I think that there are shared concerns over Iran support for terrorism, including in the Gulf where six million Indians work. So there''s a lot that unites the US and India when it comes to analysing and assessing Iranian activity," the official said.

The US is seeking to ramp up pressure on Iran to counter what the White House perceives to be a potential threat.

Last month, the US designated Iran''s Revolutionary Guard a foreign terrorist organisation, the first time the designation has been applied to a government entity.