Both benchmarks were close to two-month lows reached last week.

Traders said that ongoing oversupply and economic trends were weighing on oil.

"Headwinds (are) growing for the second half of 2016, hence our bearish oil bias," Morgan Stanley said on Monday in a note to clients, pointing to resilient US supply, falling demand for transport fuels, and oversupply by refiners, particularly in gasoline.

"As a result, oil demand from refineries is underperforming product demand by a wide margin," the U.S. bank said, adding that growing economic risks added to downside risks for crude.

A strong dollar and a fourth weekly rise in the US oil rig count also weighed on prices, traders said.

Money managers cut their net long US crude futures and options positions, which would profit from rising prices, to a four-month low in the week to 19 July, the US Commodity Futures Trading Commission said on Friday.

Libya's hopes to boost crude exports have been dealt a blow after the head of the National Oil Corporation (NOC) objected to a deal between the government and local guards to reopen key ports.