The Taliban has announced the start of its spring offensive, signalling plans for an increase in violence as the weather warms across Afghanistan, making both travel and fighting easier.

The statement comes toward the end of a month that already has been the deadliest of the year.

The Taliban leadership promised "every possible tactic will be utilised in order to detain or inflict heavy casualties on the foreign transgressors".

US-backed efforts to try to reconcile the Islamic militant movement with the Afghan government have so far failed.

Insurgents have already stepped up attacks this spring as they try to position themselves for power ahead of national elections and the planned withdrawal of most US and other foreign combat troops by the end of 2014.