UK says military operations in Libya cost 260 mln pounds

* Weapons fired will cost up to 140 million pounds to replace

* UK had said Libya cost would only be tens of mlns of pounds

LONDON, June 23 (Reuters) - Britain's military operations in Libya have cost 260 million pounds ($420 million), with expended munitions accounting for more than half the total expenditure, Defence Secretary Liam Fox said on Thursday.

Operational costs were about 120 million pounds and the replacement cost of the munitions fired was up to 140 million pounds, Fox said in a written statement to parliament.

Britain has been contributing to NATO's operation in Libya to enforce a no-fly zone since March. The government originally said it would cost tens of millions but last week Treasury Chief Secretary Danny Alexander said it could run "into the hundreds of millions" of pounds.

Earlier this week Prime Minister David Cameron said Britain could maintain its military campaign in Libya as long as needed, responding to doubts voiced by senior British military officers about how long the country could sustain its involvement.

British military leaders have complained about the pressure of fighting on two fronts, in Afghanistan and against Libyan leader Muammar Gaddafi's forces, while facing cuts in military spending to rein in a record national budget deficit. ($1 = 0.618 British Pounds) (Reporting by Tim Castle and Jodie Ginsberg)