Oil prices slid Monday, extending last week's sharp decline as the dollar strengthens on eurozone strains and record supplies in the United States add further downward pressure.
US benchmark West Texas Intermediate (WTI) for delivery in April shed $1.17 to $49.64 a barrel compared with Friday's close.
Brent North Sea crude for April lost $1.24 to stand at $58.98 in London afternoon deals.
WTI slumped 4.66 percent and Brent tumbled 2.1 percent during the week to last Friday.
"Crude oil prices extended declines (Monday)... as investors remained cautious following the ongoing uncertainty in the eurozone, while the strong US dollar currently weighed on market sentiment," said Myrto Sokou, senior research analyst at Sucden brokers.
"Crude oil inventories continue to remain fairly high, following sharp builds of crude oil stocks in the last few weeks. The continuous large increases of crude inventories verify a possible slowdown of US oil demand," she added.
Globally, crude supplies are being boosted after oil fields in eastern Libya resumed pumping to the Hariga port after a pipeline was repaired, Bloomberg News reported.
And Oman, the biggest Middle East producer outside the Organization of Petroleum Exporting Countries, plans to ramp up output to 980,000 barrels a day this year.
Crude prices lost around 60 percent of their value between June and late January owing to an oversupply in world markets, a weak global economy and a strong dollar that made oil expensive to purchase for holders of rival currencies.
The euro continued to be pressured by the dollar Monday despite eurozone ministers tentatively agreeing to extend Greece's bailout by four months.
While oil prices have won support in recent weeks on a decline in operating US oil rigs and as energy giants cut investment, markets-watchers say volatility is likely to continue for some time.
Daniel Ang, an investment analyst with Phillip Futures in Singapore, said a sustained rebound for oil prices was unlikely because of a global oversupply of the commodity, noting that US crude production has remained strong at above 9.2 million barrels per day.
He said a strike in US oil refineries has led to an excess of crude in the American market as less of the product is being sent to the facilities for refining.
Workers at three major US oil refineries operated with Royal Dutch Shell went on strike on Saturday in a dispute over safer working conditions, the United Steelworkers (USW) union said.
About 1,350 employees in three refineries and chemical plants are affected by the latest strike action, which mirrored walkouts on other sites launched earlier this month.