A massive Nato-led counter-insurgency operation is in the offing in Taliban-controlled Southern Afghanistan, prior to which villagers have started an exodus from the place, it has been reported.

The operation to clear insurgents from the southern town of Marja, in Helmand province, is expected to begin soon.

It is expected to be one of the largest counter-insurgency operations since the Afghan conflict began in 2001.

The UK defence secretary has warned of likely casualties within coalition troops during the offensive.

"Of course casualties are something that we have to expect when we are involved in these operations," Bob Ainsworth said late on Sunday.