India's imports of crude oil from Iran more than halved in June from a year ago, as refiner Essar Oil became the only remaining Indian client of the sanctions-hit country, tanker data obtained by Reuters showed.

India's imports for June fell about 60 percent on an annual basis, pointing to imports from Iran's top four customers - China, Japan, India and South Korea - of around 860,000 barrels per day (bpd) for the month, down more than a third on the year.

That would be the lowest for Iran's top four buyers since April, when big drop-offs in barrels shipped into India and Japan cut the total to 635,750 bpd, the smallest in decades.

U.S. and European Union sanctions aimed at Iran's disputed nuclear programme are costing Tehran billions of dollars per month. And U.S. lawmakers want to toughen them further, with the goal of reducing Iran's oil shipments to 500,000 bpd or less.

"The downturn year-on-year of Iranian crude imports will continue," said Praveen Kumar, who heads the South Asia oil and gas team at consultancy FGE.

"Everyone was waiting for the elections (in Iran) to happen and hoping that the new president will be more open to coming back to the negotiating table ... but we don't think there is going to be a breakthrough," Kumar said.

Western countries believe Iran's nuclear programme is aimed at making a bomb, while Iran says it is for peaceful purposes.

Iran's president-elect, Hassan Rouhani, who takes office next month, pledged in June to be more transparent on the nuclear programme but no immediate curtailment of its uranium enrichment is expected.

Indian imports from Iran dropped to 140,800 bpd in June, down 45 percent from May, data from trade sources on tanker arrivals shows.

India's imports from Iran dropped in the first half of the year to 211,400 bpd, down more than 42 percent from the same period in 2012, according to the data.

A Reuters estimate of June crude imports from Iran by Asian buyers is based on the Indian tanker data, earlier data on Chinese and South Korean oil imports, and an assumption that Japan imported about 200,000 bpd last month.

The figure for Japan, which reports its full oil import data for June next week, is close to its average daily shipments of Iranian crude for the year ended March 31.

Similar calculations and assumptions put Asia's imports of Iranian oil at about 975,280 bpd for the first half of 2013, down just over a fifth from a year ago.

Iran's share of total Indian oil imports dropped to 5.4 percent in the first half, down from more than 10 percent from last year, the tanker arrival data also showed.

Hindustan Petroleum Corp and Mangalore Refinery and Petrochemicals halted their Iranian oil purchases in April amid difficulties securing insurance for refineries processing oil from the sanctions-hit country.

Last month, Washington granted its third 180-day waiver on sanctions applied to Asian countries, including India, China and South Korea, for significantly reducing Iranian oil imports in the six months through May.

Japan won its third six-month waiver in March as part of a different review process. Japan's renewal will come up in September, while the waivers for the other Asian buyers will come up in November-December.

India imported nearly 66 percent more oil from Latin America in the January to June period as it cut its dependence on Iran. The region accounted for about 19 percent of India's overall imports, up from 12.6 percent in the same period a year ago.

Overall, Asia's third-largest economy shipped in 14 percent more oil in June than a year ago, while Indian imports for the January-June period rose 9.7 percent, the data showed. – Reuters