Afghan Border Police display bullets which were recovered from suspected militants in Helmand province on Monday. The border police seized explosives and other materials after a clash with the militants.

AFP/Kabul

At least nine people were killed on Monday in two separate roadside bombings targeting Afghan police vehicles, highlighting an uptick in attacks against security forces before the Taliban's expected spring offensive.

In the deadlier of the attacks, four policemen and two civilians were killed when a bomb ripped through a Ford Ranger pickup truck north of Kabul.

"Today morning a police patrol vehicle hit a roadside bomb blast in Qarabagh district near Kabul city, killing six people," General Mohammad Farid Afzali, Kabul police criminal investigation chief, told AFP.

The interior ministry confirmed the attack and blamed it on the "enemies of Afghanistan and insurgents", alluding to the Taliban.

Also on Monday a roadside bomb detonated against another police vehicle in the northern province of Baghlan, killing three policemen and wounding two others, provincial police chief Aminullah Amarkhil said.

There was no immediate claim of responsibility for the attacks, but roadside bombs have been a weapon of choice for the Taliban in their 13-year war against foreign and Afghan forces.

Taliban insurgents have already stepped up suicide attacks on government targets following an Afghan army offensive which began in southern Helmand province more than two months ago.

Casualties among Afghan troops and police soared after US-led troops began pulling back from the front lines. Nato's combat mission formally ended in December.

But a small follow-up foreign force has stayed on to train and support the local security forces.

Afghan forces are bracing for what is expected to be a bloody summer push by the Taliban and the government has also raised the ominous prospect of the Islamic State making inroads into Afghanistan.