The Taliban in Afghanistan has announced the start of their annual "spring offensive" against the US-backed government, vowing a nationwide series of attacks as foreign troops withdraw.

In a statement released on Saturday via email, the group promised violence that would include "insider attacks" by fighters and "special military tactics" to target international airbases and diplomatic buildings to inflict maximum casualties.

The Taliban also warned Afghans working for President Hamid Karzai's "stooge" regime to distance themselves from the government to avoid being caught up in the attacks, and called for young people not to join the police or army.

This year's "fighting season" is seen as crucial to Afghanistan's future as its much-criticised security forces pit themselves against the armed group who have fought against the Kabul government since 2001.

NATO combat operations in Afghanistan are due to end next year, and coalition commanders say that the local army and police have made enough progress to provide security and keep the Taliban at bay.

Afghanistan's fighting season traditionally begins in April or May as snow recedes from the mountains, and in recent years the Taliban have marked the occasion with a public declaration of their intent to bring down Karzai.

"Degenerate invaders"

The Taliban's latest statement celebrated the start of the NATO withdrawal, saying that "the enemy, with all its military might, has been overwhelmed and finally forced to flee from their military bases".

It added that this year's offensive, named after 7th-century general Khalid bin Waleed, would start on Sunday "in unison throughout the country ... against the transgressing invaders and their degenerate backers".

Last week a study by the Afghanistan NGO Safety Office found attacks by the Taliban and other armed groups rose 47 percent in January-March compared with the same period last year.

The United Nations has separately reported a rise of almost 30 percent in civilian casualties in the first quarter compared with the same period last year, with 475 civilians killed and 872 wounded.

But NATO insists that the war is being won, with US General Joseph Dunford, head of the International Security Assistance Force (ISAF), saying on Thursday there was "indisputable" progress towards the goal of a stable nation.

Dawlat Waziri, a defence ministry spokesman, told AFP news agency that Afghan forces were ready to take on the rebels alone.

"The Taliban are not able to fight face-to-face, so they heavily rely on roadside bombs and suicide attacks and their attacks inflict a heavy toll on Afghan civilians," he added.