The Taliban say they are starting their spring offensive this week, an annual campaign in their war against the Afghan government.

Wednesday's statement, emailed to media, says the offensive — dubbed "Azm" or perseverance in Dari and Arabic — will begin on Friday.

The Taliban say their Islamic Emirate of Afghanistan is "determined to prolong the ongoing jihad," or holy war.

In past years, spring and the melting of snow on the mountains along the border with Pakistan has marked a significant upsurge in fighting between the Taliban and NATO forces along with their local allies.

This fighting season is the first year the insurgents will face just Afghan forces on the battlefield after the withdrawal of most international combat troops at the end of last year.