Crude oil futures tumbled on Wednesday as banking troubles in Greece further highlighted the euro zone debt crisis, pulling U.S. equities lower as risk aversion hit across markets.

The European Central Bank confirmed a Reuters report that it has stopped providing refinancing to some Greek banks as they are severely undercapitalized, keeping oil and other commodity prices pinned down.

The ECB move had erased an uptick in oil prices in morning trading after U.S. data showed that domestic stockpiles rose much less than what an industry group reported late on Tuesday.

Uncertainty in Greece, already shaken by the inability of politicians to form a government, pulled the euro to near a four-month low against the U.S. dollar while Wall Street sharply slashed gains. That, in turn, provoked a further sell-off in oil futures.

"The S&P 500 turned negative, deepening the weakness in the stock market on concerns about Greece and that has speeded up the pace of oil's slide today," said Mark Anderle, broker at TAC Energy in Dallas.