Kabul: Taliban insurgents in Afghanistan announced the start of their annual "spring offensive" on Thursday, vowing a final summer of bloody attacks on foreign forces before the 13-year NATO combat mission ends.

The Islamist extremists said that the offensive, beginning on Monday, would cleanse "the filth of the infidels" from the country, and warned that Afghan translators, government officials and politicians would also be targeted.

Afghanistan's Taliban fighters have launched a 'spring offensive' against NATO forces every year since losing power in 2001. Credit:Reuters.

The "Khaibar" offensive, named after an ancient battle between Muslims and Jews, will coincide with a planned second round of elections next month to choose a successor to President Hamid Karzai, who has ruled since the fall of the Taliban in 2001.

The 51,000 US-led NATO troops still deployed in Afghanistan are set to withdraw by December, ending a long and costly battle to defeat the rebels, who launched a fierce insurgency after being ousted from power.