API reported a surge of 3.294 million barrels in the previous week.
Economists were expecting an increase of about 1.650 million barrels.
Oil prices climbed on Tuesday, bolstered by supply shortage on the market.
The West Texas Intermediate for December delivery added 89 cents to settle at 84.65 U.S. dollars a barrel on the New York Mercantile Exchange. Brent crude for December delivery increased 41 cents to close at 86.40 dollars a barrel on the London ICE Futures Exchange.
Both crude benchmarks hit their multi-year highs as the market remains tight.
"Demand for energy has rebounded sharply as economies reopen after the COVID-19 pandemic," Mark Haefele, chief investment officer at UBS Global Wealth Management, said in a note.
"At the same time, supply has been constrained by various issues including weather-related outages, integration challenges with renewables, and supply chain bottlenecks," he added.
The Organization of the Petroleum Exporting Countries and the International Energy Agency in their respective monthly reports saw oil market as being significantly under-supplied in short term.
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