A judge on Wednesday ordered Apple's chief executive Tim Cook to testify in a case brought by the US government accusing the tech giant of conspiring to raise e-book prices.

Federal Judge Denise Cote issued the order after a teleconference with attorneys from Apple and the US Justice Department.

She said the government may take a deposition from Cook, which could be used in a trial, overruling objections from the company. The order said Cook must testify "for the reasons stated on the record at the teleconference," without elaborating.

Apple lawyers recently argued that Cook should not be compelled to testify because he was not involved in the deals with publishers led by Apple's former chief Steve Jobs before his death in 2011.

Cook "has no unique personal knowledge of relevant facts," an Apple petition said, adding that the new CEO was not discussed in any prior court filings and that at least 11 other Apple executives would be testifying.

The lawsuit filed last year accuses Apple and five publishing firms of colluding to fix prices on electronic books, to change a model developed by Amazon.

Each of the publishers settled with the government, agreeing to cease any special deals with Apple for e-books.

US officials allege the price-fixing scheme was aimed at ending a discounting effort by Amazon, which sold most e-books at $9.99 until Apple's new pricing plan was forced on the retail giant.

The move almost instantly raised the prices consumers paid for e-books to $12.99, $14.99 or higher, according to the US complaint.