Brent slipped below $86 and is headed for its fourth weekly loss in a row, despite climbing by almost a dollar earlier, as investors see no respite in a four-month rout sparked by ample supply and slowing demand from Europe to China.

Brent has lost more than 20 per cent of its value since June and was dragged down earlier in the week by signals from key Opec members suggesting the group was unlikely to intervene on prices. A downward revision in global oil demand for 2015 by the International Energy Agency further depressed the market.

Front-month Brent was down 12 cents a barrel at $85.70 at 0645 GMT. The November contract expired on Thursday, settling up 69 cents at $84.47 a barrel.

US November crude, on track to post its third weekly decline, edged down two cents to $82.68 after settling 92 cents higher in the previous session.

"Most of the traders in the market are probably trading from the short side, so when we see a little bit of buying, we often think that we'll see a little bit of a short-covering rally," said Ben Le Brun, a market analyst at OptionsXpress.

"If that's the case, there might be some weakness still to play out."

Leading oil analysts across Wall Street have raced this month to slash their price forecasts by as much $12 a barrel as old assumptions about Saudi Arabia's readiness to defend a $100 crude are radically revised.

"There's a glut of supply on the market at the moment. A lot of North American production coming up is not helping that side of the equation," Le Brun said.

"Outside of the US, there's not a lot of demand at the moment. Unfortunately, that's a story that could play out at least into early 2015, until we see some further central bank support," he said, referring to possible stimulus measures from the European Central Bank, Bank of Japan and People's Bank of China.

Technical analysts said US oil prices may slump further if they fall below critical support at $80.

"In terms of magnitude we now see more upside risk to oil prices than downside risk, but we would not rule out another several weeks of price weakness, and a possible run at $75 a barrel WTI in the meanwhile," CIBC analyst Katherine Spector said in a note.

Crude inventories in the United States surged by nearly 9 million barrels last week as refineries cut output and imports inched up, data from the Energy Information Administration showed on Thursday.

Gasoline inventories fell 4 million barrels to their lowest level since November 2012, compared with analyst expectations in a Reuters poll of a drop of 1.2 million barrels. - Reuters