New York's main contract, West Texas Intermediate (WTI) crude for delivery in July, was down 33 cents to $86.20 per barrel while Brent North Sea crude for July shed 35 cents to $101.52 in morning trade.
WTI crude had yesterday plunged USD 1.29 to $86.53, its lowest closing level since October 20, while Brent crude shed USD 1.60 to hit a five-month low of $101.87.
The declines in both contracts in the past week capped the worst month for oil prices since December 2008, with futures falling 17 percent in May, Dow Jones Newswires reported.
"Oil prices continued to fall extremely heavily.... At the moment, sentiment seems be pricing in a dark future of major economic discontinuities," said Barclays Capital in a commentary.
The oil market has been pounded in recent days along with equity markets amid fears over Spain's troubled banking sector.
Demand has also been hurt by the plunging euro, making dollar-priced crude more costly for eurozone countries.
Concerns were further exacerbated by weak US economic and jobs numbers as well as the weekly oil stockpiles report showing an increase of 2.2 million barrels.
Stockpiles in the US, the world's largest oil-consuming economy, now stand at the highest level in 22 years for this time of the year.