Brent crude surged to a six-month high on Wednesday as western countries prepared to attack Syria, raising concerns over the security of oil supplies across the Middle East, which pumps a third of the world's oil.

The United States and its allies are readying for air strikes against the forces of President Bashar al-Assad, blamed for poison gas attacks last week. But the timing of any action was unclear.

A prolonged outage at several Libyan oilfields also underpinned prices, with Brent gaining 4 percent and the U.S. benchmark 3 percent so far this week.

Brent was at $115.83, up $1.47 at 1214 GMT after earlier reaching a six-month high of $117.34 a barrel. U.S. crude rose $1.45 to $110.46, after hitting an intraday peak of $112.24 - its highest since May 2011.

French bank Societe Generale says Brent could spike to $150 if the conflict in Syria spreads and disrupts supply in the region. Syrian oil output itself is not a factor: it has fallen to 50,000 barrels per day (bpd) from around 350,000 bpd when the unrest started two years ago.

"The concern is that an attack on Syria will reverberate through the region, increasing the spillover into other countries and possibly resulting in a larger supply disruption elsewhere," said Michael Wittner, oil analyst at the bank.

Over the coming days, Brent could surge to $125, either in anticipation of the attack or in reaction to its start, he said.

Oil supply from OPEC producer Libya has already been reduced to a trickle after an armed group shut down a pipeline linking its largest western oilfields to the ports.

Total Libyan oil output amounts to just under 200,000 bpd, compared with pre-war levels of around 1.6 million bpd, according to a Reuters estimate, the worst disruption since the civil war in 2011.

"While the headlines are currently on Syria and the speculative extrapolations about the Suez canal, the Gulf of Iskenderun or the Strait of Hormuz, there is currently a supply disruption in Libya as real as in 2011," Olivier Jakob of Petromatrix in Zug said.

Oil could also get a boost if the U.S. Federal Reserve decides to start paring its bond purchases later than the anticipated September timeline.

Investors awaited weekly oil inventory data from the United States later in the day for clues on demand in the world's top consumer.

U.S. crude stocks rose last week while gasoline inventories declined and distillate stocks increased, data from industry group the American Petroleum Institute showed. (Additional reporting by Florence Tan and Manash Goswami in Singapore; editing by Jane Baird and Keiron Henderson)