Crude prices extended last week's losses on Monday as the International Energy Agency warned of a further fall in oil investment.
US benchmark West Texas Intermediate (WTI) for delivery in December fell 62 cents, or 1.4 percent, to $43.98 a barrel on the New York Mercantile Exchange.
Brent North Sea crude for December delivery closed at $47.54 in London, down 45 cents (0.9 percent) from Friday's settlement.
Last Friday, oil prices stretched losses into a second week, with WTI down $2.02 for the week and Brent shedding $1.94.
IEA executive director Fatih Birol said Monday that ample oil supplies likely would extend into the middle of next year, while investment is expected to decline further because of persistently low prices.
He told an energy conference in Singapore that oil investment is already down 20 percent worldwide this year and another drop is likely in 2016.
That would mark the first time oil investment has declined for two years in a row since the mid-1990s.
Commerzbank analyst Carsten Fritsch said that Brent has been trading around $48 for the past eight weeks, apart from a brief surge in early October.
"Things are unlikely to be any different this week either, for the fundamental situation remains unchanged: the market is significantly oversupplied at the moment, as overflowing stocks of crude oil and products -- above all in the United States -- confirm time and time again," he said.