Iranian crude oil deliveries to India were up in January more than 37 percent from December levels to 550,000 barrels per day, making India one of Iran's largest energy customers.
That increase came as Western governments target Iran's finances with sanctions, making it difficult for Tehran to process crude oil payments.
Trevor Houser, a partner at the economic research company Rhodium Group, told The Wall Street Journal he "didn't think India is insulated at all" from the economic sanctions.
"So the question is just how long Tehran is willing to sell on credit and how long Tehran can go without hard currency," he said.
New Delhi this week said it was processing at least some of its crude oil payments through an Indian bank in an account set up by Tehran. India at one point was receiving crude oil from Iran on credit in part because of sanctions pressure.
The Journal, citing unidentified sources, said Iranian crude oil deliveries to South Africa increased during the reporting period. Crude oil shipments to China, however, fell 50 percent to around 250,000 bpd because of what the newspaper described as a pricing dispute.