Oil prices slipped on Wednesday after an unexpectedly high build in US crude inventories, further evidence of a persistent global oversupply.

US crude oil stockpiles rose 4.6 million barrels in the week to Sept. 25, the American Petroleum Institute (API) said, well above a modest increase of 102,000 barrels that analysts polled by Reuters had forecast.

Investors awaited official weekly inventory figures from the US government's Energy Information Administration (EIA) due later on Wednesday to see if they confirmed the API data.

US crude, also known as West Texas Intermediate or WTI, was 10 cents lower at $45.13 a barrel by 0900 GMT, on course to end September down 11 percent.

Brent crude oil was unchanged at $48.23 a barrel, heading for a near 9 percent fall this month.

Brent traded in a very narrow 60-cent range on Wednesday, its narrowest since May 2014, partly reflecting low volume ahead of the week-long Chinese National Day holiday starting on Thursday.

"We continue to see this as range-bound for both WTI and Brent," said Daniel Ang, analyst at Phillip Futures. "We do not believe this can break without (a) fundamental shift."

Inaction by the world's largest crude exporter Saudi Arabia to prop up prices has helped it build market share, a Reuters analysis shows. Saudi exports to Asian and European consumers reached multi-year highs in the first half of the year.

Saudi Arabia is banking on a rise in world oil demand and slower growth in non-Opec oil supply, meaning it is unlikely to change its stance on not cutting production any time soon.

However, other Opec members are trying to gather support for a change in tactics to lift prices. Kazakhstan's energy minister said on Wednesday he had been approached by Opec members to coordinate policies to stimulate oil price growth. He did not identify the Opec members concerned.

Kazakhstan is not a member of Opec. It is the second-largest oil producer among ex-Soviet countries after Russia.

The chief executive of commodity trader Vitol, Ian Taylor, said also in Kazakhstan he was seeing signs that global oil supply and demand were beginning to balance out.

"This is beginning to happen ... Demand is very strong, you can see US production beginning to come down," he said, declining to give a forecast for oil prices.-Reuters