(Adds details, background)

NAIROBI, Aug 27 (Reuters) - Kenya Airways, which is being renationalised to save it from mounting debts, saw its first-half pretax loss more than double from a year earlier to 8.56 billion Kenyan shillings ($83 million), its results statement showed on Tuesday.

Kenya's parliament voted in July to renationalise the loss-making airline, which is labouring under a mountain of debt and has had three changes of chief executive in the past five years as it struggles to compete with regional rivals.

The government plans to buy out minority shareholders including Air France-KLM's 7.8% stake.

The carrier said its first-half revenue rose 12.2% from a year earlier to 58.55 billion shillings, which Chief Financial Officer Hellen Mathuka attributed to the launch of new routes and more frequent flights.

However, the airline's operating costs rose to 61.45 billion shillings, from 53.22 billion shillings in the same period last year, Mathuka said. That was partly due to two Boeing 787 planes that had been sub-leased to Oman Air being returned during the first half of the year, she said.

The government will take at least 21 months to take back full control of Kenya Airways, buying out minority shareholders and converting shares held by banks into Treasury bonds, a lawmaker briefed on the transaction told Reuters in July.

($1 = 103.2000 Kenyan shillings) (Reporting by George Obulutsa Writing by Maggie Fick; Editing by Susan Fenton)