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.

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.

A small number of US troops may stay on from next year on a training and counter-terrorism mission, but the Taliban warned that the insurgency would continue against even a few thousand US troops.

The Taliban "insists on the unconditional withdrawal of all invading forces... and sees the continuation of its armed Jihad (as) imperative to achieving these goals," said an English-language statement on the group's website.

"If the invaders or their internal stooges believe that reducing the number of foreign forces will dampen our Jihadi fervour then they are sadly mistaken."

It added that attacks during the coming "fighting season" would target US military bases, foreign embassies and vehicle convoys, as well as the Afghan government.

- 'Barbaric invaders' -

"The days of... the barbaric invaders (on) the pure soil of our country have come (to a) close, Allah willing, due to your 13-year Jihad and sacrifices," it told its fighters.

Afghanistan's fighting season traditionally begins in April or May as snow recedes from the mountains, and the Taliban mark the occasion with an annual declaration to attack foreign forces and unseat the Kabul government.

"The main target of the current year's blessed Jihadi operation shall be the foreign invaders and their backers under various names like spies, military and civilian contractors and everyone working for them like translators," it said.

The offensive will consist of suicide bombings, "insider attacks" by Afghan soldiers and complex assaults on military facilities.

"Such war techniques which shall inflict maximum losses on the invaders while preventing corporeal and financial losses on the ordinary civilians," it added, though insurgent attacks often kill non-combatants.

The Taliban had vowed to disrupt the first-round of presidential elections on April 5, but they failed to launch a major attack on the campaign or on polling day.

Thursday's statement did not mention the election, which the insurgents have often dismissed as a US plot to install a puppet government.

Former foreign minister Abdullah Abdullah and ex-World Bank economist Ashraf Ghani are due to compete in a run-off vote in June after neither won more than 50 percent in the first round.

A potentially violent second-round election could be avoided by a power-sharing deal between the two candidates, but both Abdullah and Ghani have so far dismissed the possibility.

The next president is likely to sign a long-delayed deal with Washington to allow between 5,000 and 10,000 US troops to remain in Afghanistan after December.

He may also revive efforts to start negotiations with the Taliban. Previous steps to begin a peace process ended in failure.