Oil fell more than $2 a barrel to a five-year low in Asian trade on Monday, extending a steep sell-off after OPEC decided not to cut production last week, keeping markets well supplied.
Both US crude and Brent have fallen for five straight months, oil's longest losing streak since the 2008 financial crisis.
"Just when the market was thinking that a 4-year low for crude oil was bad enough, we have hit a 5-year low after the OPEC meeting," Phillip Futures analysts said in a note.
US crude tumbled to near $64, dragging Brent down below $70, after Saudi Arabia's oil minister told fellow OPEC members last week that they must combat the US shale oil boom.
"They (OPEC) can get by at $60 a barrel, but that price would knock out a fair whack of the competition - much US shale oil for example - as well as put investment in future capacity growth firmly on the back-burner," ANZ analysts said in a note.
"They're playing the long game, banking that others can't."
Brent hit a low of $67.82 a barrel, the lowest since October 2009, and was down $1.85 at $68.30 a barrel by 0447 GMT. US crude was down $1.61 at $64.54 a barrel, after earlier slipping to an intraday low of $64.10, the lowest since July 2009.
Oil-producing countries from Iraq to Nigeria are revising their 2015 budgets to reflect lower prices.
Iran refrained from protesting against OPEC's decision to retain its production ceiling to maintain group solidarity, even though the move will not benefit all members, Iranian oil minister said in local media reports.
Slower than expected growth in China's manufacturing sector may add further downward pressure on oil. China's official Purchasing Managers' Index (PMI) slipped to 50.3 in November, a government study showed on Monday, lower than analysts' forecasts of 50.6.
"It's not too bad a miss, but probably won't help (oil) too much," Ric Spooner, chief analyst at CMC Markets in Sydney said.
"It's best not to try to pick bottoms at this stage."
Spooner said the next support levels for Brent are at $68 and $64 a barrel. If West Texas Intermediate crude futures fall below $63.90 a barrel, the next support level would be at $50, he said.