Oil fell in Asian trade amid lingering worries over energy demand in the United States, the world's biggest oil user, analysts said.
New York's main contract, light sweet crude for November delivery, dropped 21 cents to $65.81 a barrel.
Brent North Sea crude for November delivery was off 39 cents to $64.72.
Concerns over US energy demand are re-surfacing after data released on Friday showed orders for durable goods fell 2.4 per cent in August against market expectations for a rise of 0.4 per cent.
Durable goods are those likely to last three years or more, such as cars and appliances, and represent a key segment of the manufacturing sector.
"International economic news was generally on the disappointing side of market expectations, leaving metal and oil markets lacking impetus," analysts from the Commonwealth Bank of Australia said in a report.
"The US economic data failed to allay concerns that US oil demand remains tepid," they said.
Looking ahead, investors will be watching out for a slew of US data due out this week including the key non-farm payrolls figures and unemployment rate for September expected to be released on Friday.
"The key focus of the week is, as always, are the non-farm payrolls and unemployment reports due Friday," Singapore's DBS bank said in a report.