KABUL (AFP) - The Afghan Taleban announced Wednesday they will launch their annual "spring offensive" from April 24, vowing attacks across the country at a time when US-led foreign forces are pulling back from the frontlines.

The militants have already stepped up attacks on government and foreign targets in recent weeks, with Afghan security forces bracing for what is expected to be the bloodiest fighting season in a decade.

It will be the first fighting season in which Afghan security forces will battle the insurgents without the full support of Nato combat forces.

"The Islamic Emirate is going to launch the spring operations under the inspirational name of 'Azm' (determination) at 5am on 24th April 2015," the Taleban said in a statement, using their official name.

"The main targets of these operations... will be the foreign occupiers, especially their permanent military bases... officials of the stooge regime, their military constellations, especially their intelligence, interior ministry and defence ministry officials."

Nato's combat mission formally ended in December but a small follow-up foreign force has stayed on to train and support local security personnel.

President Barack Obama last month announced a delay in US troop withdrawals from Afghanistan, an overture to the country's new reform-minded leader, President Ashraf Ghani.

Hosting Ghani at the White House for their first presidential face-to-face meeting, Obama agreed to keep the current level of 9,800 US troops until the end of 2015.

The Taleban, who have waged a deadly insurgency since they were ousted from power in late 2001, warned that the announcement would damage any prospects of peace talks as they vowed to continue fighting.

An upsurge in attacks in recent weeks has taken a heavy toll on ordinary Afghans.

In the first three months of 2015, civilian casualties from ground fighting were up eight per cent on the same period last year, a recent UN report said.

But in their statement, the Taleban said their "top priority will be given to safeguard and protect the lives and properties of the civilian people" during the spring offensive.

The Afghan government and Nato are yet to respond to the Taleban's announcement. In the past they rejected these offensives as propaganda.