The U.S. has been pushing its allies to halt their import of Iranian oil ahead of the November deadline.
In May, Trump controversially pulled America out of the nuclear deal Obama agreed with Iran in 2015.
Iran's oil exports could fall by as much as two-thirds by the end of the year because of the U.S. sanctions, putting oil markets under strain amid supply outages elsewhere in the world.
In July, Beijing lifted monthly oil imports from Iran by 26 percent.
Francis Fannon, the assistant secretary of state for the Bureau of Energy Resources, was recently in China to discuss sanctions, according to a State Department spokesperson.
As part of the withdrawal, the US will on Monday begin to sanction Iranian transactions with USA dollar banknotes; trade in gold and precious metals; direct or indirect sales of graphite, raw or semi-finished metals; and Iran's automotive sector.
To keep its shipment flowing Iran has offered insurance for oil cargo and ships to supply oil to India, its top oil client after China. Global sanctions were lifted under the pact between world powers and Tehran in return for curbs on Iran's nuclear program.
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Washington imposed a series of additional sanctions on Iranian entities and individuals, as well as foreign companies in Iran, squeezing Iran economically.
The activity extended to the Strait of Hormuz, a strategic waterway for oil shipments the Revolutionary Guards have threatened to block.
Later in the year, on November 4, all countries must stop importing Iranian oil or they, themselves, will face financial punishment from Washington.
Increased US hostility has also driven a run on Iran's currency, which has lost around two-thirds of its value in six months.
More than half-a-dozen current and former officials told the Reuters news agency that the campaign is meant to work in concert with US President Donald Trump's push to economically throttle Iran by re-imposing tough sanctions.
State refiners accounted for about four-fifths of the imports; Indian Oil Corp along with its unit Chennai Petroleum Corp imported approximately 300 000 bpd oil, the preliminary data showed.
Tamar Essner, an analyst at Nasdaq Inc., said there are "no signs whatsoever that this trade war is going to clear up anytime soon", and that has "caused investors to continue to trim net length, take profits, and de-risk that position with the sense that oil's upside is limited unless there's material reduction in Iranian barrels".