According to Reuters, India's oil imports from Iran in May rose by 10.2 percent from the previous month and were about 45 percent more than a year ago, the data showed.
Iran's oil exports hit 2.7 mbd in May, representing a record high since the lifting of international sanctions on Tehran in 2016.
Indian state refiners had cut oil imports from Iran in the 2017-18 financial year due to a dispute over development rights for a giant gas field.
However, the refiners had drawn plans to raise imports in the current fiscal year from April after Iran agreed to steep shipping discounts.
India, the world's third-biggest oil importer and consumer, is Iran's second-biggest customer behind China.
Nayara Energy, formerly known as Essar Oil and in which Russia's Rosneft holds a majority stake, has started reducing Iranian oil imports from this month, sources said.
In May, Nayara was the biggest Indian buyer of Iranian oil, receiving more than 200,000 bpd, the data showed.
Reliance Industries, owner of the world's biggest refining complex, is preparing to halt imports of Iranian oil from October or November, sources said.
India's Petroleum and Natural Gas Minister Dharmendra Pradhan said last week that the country has adopted a 'wait and watch' policy pending clarity on the impact of US sanctions.
In the first five months of 2018 India imported 583,500 bpd from Iran, about 6.5 percent more than a year ago, data from the sources showed. The sources declined to be identified because they are not authorized to speak to the media.
Iran remained the third-biggest oil supplier to India last month behind Iraq and Saudi Arabia, the data showed, with the United Arab Emirates having replaced Venezuela as the fourth-biggest supplier.
Overall, India imported 4.34 mbd in May, about 2.8 percent higher than a year ago.
From June, India's oil imports from Iran could drop because at least two refiners — accounting for about 35 percent of Indian refining capacity of around 5 mbd — are preparing to curb purchases under pressure from the US sanctions.
On May 8, US President Donald Trump said that the United States was pulling out of a 2015 international nuclear deal with Iran and would impose new sanctions seeking to reduce the country's oil shipments.