President Donald Trump may count Indian Prime Minister Narendra Modi among his international allies, but New Delhi is smarting over unexpected United States' decisions it sees as ignoring the interests of an increasingly close partner.

The Trump administration this week said it would start to sanction countries that do not comply with its orders to stop buying oil from Iran, demanding that eight governments — including India and China — end all imports when six-month waivers run out next week.

The move, which triggered a hike in global oil prices that could disproportionately hit poorer Indians, came just as Modi was campaigning for a new mandate in ongoing, multi-phase elections.

The Iran diktat followed Trump's announcement in March that India, along with Turkey, would no longer enjoy a preferential trading status for a wide range of manufactured goods.

Trump, who has rocky relations with the leaders of numerous Western allies, has publicly highlighted his bond with Modi, a Hindu nationalist who shares the US president's hawkish stance on radical Islam.

India's main opposition Congress party quickly seized on the Iran sanctions to attack Modi. Its spokesman Randeep Singh Surjewala tweeted that the Indian leader is “sitting as a mute spectator over the country's oil needs and security".

Trump, while popular among much of the Hindu right, has also drawn resentment in India over viral reports that he mimics Modi's accent in private — a far cry from the reverential treatment US presidents since Bill Clinton have shown Indian leaders.

'Surprised and disappointed'

An Indian official, speaking on condition of anonymity, described New Delhi as “surprised and disappointed” by the decision on Iran, saying the Trump administration had sent a message in March that India's cuts in imports were sufficient to be granted a fresh waiver.

“We thought that, as a major defence and strategic partner, the United States would take into consideration our concerns,” the official said.

Trump is seeking to eliminate Iran's top source of revenue in a bid to curb the clerical regime's regional clout, including its backing of Shia militants.

India is the world's third-largest oil importer. The official said the South Asian nation has cut Iranian oil from 17 to five per cent of its total crude imports and had also ended oil purchases from Venezuela, succumbing to US pressure as Trump tries to oust leftist President Nicolas Maduro.

“We did this not because we agree with the US, but because we are strategic partners,” he said.

Similarly, the official said the Trump administration ignored a detailed proposal from New Delhi when it announced it would scrap its designation in the Generalized System of Preferences, which grants favourable access to goods from developing countries.

India is proposing a 90-day delay in implementation as the government cannot make a counter-proposal under laws that forbid policy decisions during elections, the official said.

Another rift could come up as India — a Cold War partner of the Soviet Union turned major buyer of US defence equipment — finalises its purchase of Russia's advanced S-400 missile system.

Defense Minister Nirmala Sitharaman recently told AFP that India has been “heard and understood” by the United States, which imposed sanctions on China and has warned Nato ally Turkey over buying the S-400.

Years of growing ties

Few expect India and the US to drift apart significantly, let alone return to their Cold War estrangement, with the major parties in both democracies broadly supporting a strong relationship.

And Trump pleased Modi in February by backing India's air incursions into rival Pakistan, home to virulently anti-Indian militants, in response to an attack on Indian forces in divided Kashmir.

Tanvi Madan, a fellow at the Brookings Institution and director of its India Project, said that this week's sanctions decision for the United States “is about Iran, not about its approach to India".

"But I think that India will feel once again that it will be collateral damage," she said.

“It reinforces the sense in India that the US isn't factoring in Indian interests when it comes to these decisions, yet they affect India.”

Madan added that New Delhi was less concerned about oil than with preserving a relationship with Tehran, even while developing close ties with Iran's rivals Israel and Saudi Arabia.

India is hoping to keep working with Iran on its Chabahar port, through which New Delhi hopes to send supplies to war-torn Afghanistan, cracking the landlocked country's reliance on Pakistan.

Madan said there was one wild card that could shake up US-India ties — if China seized on the rift and offered reconciliation with India by making concessions on the Asian giants' myriad disputes.

“Will that happen? I find it hard to believe the Chinese will get around to doing it,” she said. “But the possibility exists."