Oil prices plunged 5 percent to their lowest levels this year on Wednesday as U.S. crude inventories surged much more than expected to a record high, stoking concerns a global glut could persist even as OPEC tries to prop up prices with output curbs. [...]

The big daily price slide could signal a steep downward move if speculators are beginning to unwind long positions in crude oil, which were close to a record, traders and analysts said.