The Taliban's reclusive leader says insurgents will increase the number of insider attacks against coalition and Afghan forces, which have resulted in the deaths of at least 52 coalition troops so far this year.

In a Wednesday emailed statement congratulating Muslims on the eve of the Eid al-Adha holiday, Mullah Mohammad Omar said this had proven to be "the most effective stratagem."

He urged "every brave Afghan in the ranks of the foreign forces and their Afghan hirelings ... to strike them."

More than 50 Afghan soldier and policemen have died this in attacks by their own colleagues.

The surge in insider attacks is throwing doubt on the capability of the country's security forces to take over from international troops ahead of a planned handover to the Afghans in 2014.