The Defence Secretary, Dr Liam Fox, has said "it's impossible to say" how long Nato operations in Libya will last.

Speaking as the allied operation, which began with airstrikes by French jets, enters its 100th day, Dr Fox told today presenter John Humphrys: "Until the violence against the population stops we have to continue our commitment under the UN resolution 1973.

"I think that increasingly there comes a question now about the core support around Gaddafi, because it's clear the international community has both the capability and the will to continue what we've started. When will they realise that investing in Gaddafi is not a smart move and that the best way is to see an accommodation with the international community and the opposition forces."

Nato has now flown 12,000 sorties, including 5,000 attack missions, and hit more than 2,400 targets since launching strikes against Libya 100 days ago on Sunday under a United Nations mandate to protect civilians.

The British military alone has damaged or destroyed more than 500 targets, including 240 tanks, armoured vehicles, artillery and rocket launchers, and more than 150 buildings, including bunkers and ammunition stores.

The MoD said on Thursday that the UK's part in the Nato operation is costing taxpayers £43 million per month.

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