Dated Brent has been assessed below the psychologically-important $50/barrel mark for the first time since January, Platts data showed Monday. Monday's assessment of $49.145/b, was the lowest since January 30, when it was assessed at $47.855/b.

The weakness in the Dated Brent market reflected a broader weakness across the crude oil complex, which saw September ICE Brent Futures down by $2.59/b at 16:30 London time compared to the same time Friday.

"We're in the downside of demand season, so the market is collapsing right now," Price Futures Group analyst Phil Flynn said. Concerns over the possibility of slowing Chinese oil demand has also been weighing on oil futures, Flynn said.

China's financial information provider Caixin Monday said the country's general manufacturing PMI came in at a two-year low of 47.8 in July, down from 49.4 in June.

The physical North Sea crude oil market was also depressed by a growth in physical supply in the new Forties program, which saw loadings in September rise to 13.8 million barrels, compared to only 10.8 million barrels in August.

"I guess the BFOE programs are a little longer, particularly Forties," one trader said. "There are five more Forties cargoes in September, so I suppose that is bearish."