Attacks in Kunduz increased after NATO shifted supply routes to run through northern border states [Gallo/Getty]

A suicide bomber has struck at a police checkpoint in northern Afghanistan, killing one officer and injuring three civilians.

The attacker detonated his explosives as he was stopped at a checkpoint in the centre of Kunduz, Abdul Rahman Sayedkhaili, the chief of police in Kunduz province, said on Thursday.

The bombing is the second suicide attack in less than a week in Kunduz city, a major agricultural and marketing centre that controls one of the main highways into neighbouring Tajikistan.

On December 19, insurgents stormed an army recruitment centre in the city, killing four Afghan army soldiers and five police in a day-long gun battle.

The Taliban have increased their attacks in Kunduz after NATO shifted from using a supply route through Pakistan to ones running through the former Soviet states that border northern Afghanistan.

NATO has increased its forces and pressed hard against the Taliban and other fighters in the Taliban's southern strongholds, and coalition officials say NATO troops have made progress there.

Increased attacks

But violence has increased elsewhere in the country, with bombings and targeted assassinations that aim to discourage Afghans from cooperating with the federal government in Kabul.

In the latest such attack, fighters shot and killed the head of the local council in the Shindand district in the western province of Herat in his car, Noor Khan Nikzad, the spokesman for the Herat provincial police, said.

The man's son who was in the car with him was also killed.

Separately, a roadside bomb struck a tractor pulling a cart loaded with passengers on Wednesday, killing a child and wounding nine people in the Garm Ser district of Helmand province, Daoud Ahmadi, spokesman for the provincial governor, said.

The civilian deaths are particularly troubling for the Afghan government and its NATO allies.

A clash between NATO forces and fighters earlier this week left five civilians dead in Helmand, prompting criticism from the provincial government.

While NATO says it has been exercising utmost care to avoid such casualties, a UN report this month said that Afghan civilian casualties surged by 20 per cent in the first 10 months of 2010, compared with the same period a year earlier.

The UN report also found that civilian casualties attributed to NATO and pro-government forces dropped by 18 per cent compared to the first 10 months of 2009, findings which the Taliban quickly denied Wednesday as biased and exaggerated.

Meanwhile, NATO said that a Taliban leader involved in the December 19 attack on an Afghan army bus in Kabul was killed in an airstrike in central Ghazni province on Wednesday.

NATO identified the Taliban leader as Abdul Hai, and said two other fighters were also killed in the strike that targeted an alleged bomb-making cell.