As the conflict in Afghanistan shows no sign of abating, local humanitarian workers are finding themselves increasingly in the line of fire – or at the mercy of violent criminals.

The latest such attack came early on Tuesday, when unidentified gunmen shot dead nine Afghan employees of the Czech NGO People in Need in northern Afghanistan.

Ross Hollister, country director for People in Need, said the attackers stormed the organisation’s guesthouse in the district of Zari, in Balkh province, at around 1.30am, where they killed two drivers, two security guards and five project workers, including eight men and one woman. Two others survived the attack.

Hollister said the victims had been killed “execution style”, with several of them shot as they slept in bed. The organisation has suspended its Afghanistan operations temporarily.

The incident adds to grim statistics that make Afghanistan by far the deadliest country in the world for aid workers. According to the UN, 57 aid workers were killed in Afghanistan last year.

This year is set become even more brutal, with 26 aid workers killed and 17 injured so far, according to statistics released by Acbar, a coordinating body for NGOs in Afghanistan.

“Killing an aid worker is killing support for the Afghan population,” Justine Piquemal, Acbar’s director, said. “There [has been] a real difference from last year – an awful and real increase.”

As international NGOs gradually scale down activities in Afghanistan, their local staff have remained in some of the country’s most dangerous regions.

Hollister said People in Need had worked for 12 years without security incidents in Balkh province, which is not a hotbed of insurgency, “so it [the attack] really came out of nowhere”.

However, Zari district is ethnically diverse, with deep-running, political faultlines and armed groups belonging to various factions.

While its motives are unclear – and the Taliban has denied responsibility – the attack reveals the complex set of risks that face aid workers in Afghanistan, where ideology, local politics and historic grievances all play a part, as does simple criminality.

“What we can say with clarity is that humanitarian workers are more vulnerable,” said Mark Bowden, the UN’s humanitarian coordinator in Afghanistan. “They work on the frontline in many cases. Secondly, they are more noticeable in terms of what they do in the community, and in some instances, some groups believe they are on the wrong side.”

He added: “If you’re working for an international organisation, people know that you have international backers, and you can be an easier commercial target or criminal target.”

Apparent disagreements within the Taliban about who should be regarded as civilians are also a factor. Senior Taliban leaders are said to have a broader definition than some younger militants of which humanitarian workers deserve protection.

However, the Taliban generally seemed to approve of the killing last December of 12 deminers in Helmand province, as well as the kidnapping of 19 deminers in April. The mine clearers worked for Sterling, an American commercial contractor, which is likely to have classified them as foreign agents in the eyes of the Taliban.

The Taliban also claimed the attack on a hotel in Kabul on 13 May, which killed 14 civilians, including six aid workers, on the basis of targeting “foreign dignitaries”.

Mualem Rahmatullah, father of Save the Children worker Rafiullah Salihzai. Photograph: Andrew Quilty

Following the attack, Georgette Gagnon, the UN’s human rights director in Afghanistan, said the Taliban’s actions could amount to war crimes. “Taliban statements on avoiding civilian casualties ring hollow when we set them against the latest killings,” she said.

With cash-strapped militants looking for funds, kidnappings are also becoming more common. Acbar said 40 aid workers have been abducted so far this year.

The bodies of five of them, who worked for Save the Children, were found in April in the mountains of Uruzgan province, 40 days after they had been kidnapped.

Relatives of the victims said the kidnappers first demanded the release of five Taliban prisoners. When the authorities refused, they demanded the equivalent of £400,000 in ransom, which the families didn’t have.

“I was proud that my son worked for his own people … He never thought it was dangerous,” said Mualem Rahmatullah, the father of 27-year-old Rafiullah Salihzai, in a recent interview at his home in Tarin Kot.

Mohammad Tawoos Penham, whose brother’s corpse was found with one eye gouged out and bullet wounds to his chest, buttocks and back, urged humanitarian organisations to continue their work, despite the dangers.

“[The] Taliban and terrorists don’t want Save the Children to continue their activities in Uruzgan,” he said, “but they should help the children who are still here.”

He asked organisations to provide better security, such as armed guards, for their local staff. But equipping NGOs with that level of security would defy their purpose in the first place, said Bowden. “They wouldn’t participate in the community and do what they set out to do,” he said.

People in Need’s Hollister agreed: “The cornerstone of our security here is acceptance by the communities where we work. We remain neutral, impartial and not religious. But on some level there is nothing you can do.”