Contract workers for the US State Department improperly viewed Democratic presidential candidate Senator Barack Obama's passport records three times this year in what his campaign called "an outrageous breach" of his privacy.

The State Department said its initial assessment was that three workers in separate offices looked at the records out of "imprudent curiosity" rather than any political motivation but that it had requested an investigation into the matter.

The incidents, which occurred on January 9, February 21 and March 14, were quickly reported to lower-level State Department officials but only came to the notice of its senior management when a reporter e-mailed spokesman Sean McCormack on Thursday.