The number of civilian casualties in Afghanistan has decreased for the first time in six years, said the UN, but targeted killings by insurgents – particularly of women, girls and government employees – increased dramatically.

In its annual report on the protection of ­civilians in armed conflict, the UN recorded 2,754 Afghan civilians being killed in 2012, a decrease of 12% compared with 2011, and 4,805 being injured, a slight rise.

Improvised explosive devices laid by insurgents were responsible for 81% of casualties, with a 9% rise in the number of civilians killed or injured by IEDs.

"The decrease in civilian casualties [the UN] documented in 2012 is very much welcome. Yet the human cost of the conflict remains unacceptable," said Jan Kubis, the UN envoy to Afghanistan.

The overall decline was attributed in part to one of the worst winters on record impeding fighting. Fewer suicide bomb attacks and a fall in the number of air strikes also helped ease 2012's death toll.

But the report showed a 20% increase in the number of women and girls killed or injured. Deliberate targeting by the Taliban and other insurgents also tripled in 2012, said the UN. Most were hit while in their homes or working in fields.

Of the 854 female casualties, 504 were a result of insurgent attacks, while foreign and Afghan troops were responsible for 155 deaths and injuries. Tragically, however, more than 200 female casualties were unattributable and could only be explained by the UN as a result of "the explosive remnants of war".

Afghans responded to the findings with a mixture of positivity and caution.

"I think there is a sense that our own forces are doing more work" said Farzad Samandari, a 20-year-old student from Kabul. "It seems Afghan forces are far more willing to sacrifice themselves for their own country against insurgents, which is important – that's something you cannot really expect from foreign forces."

Elsewhere, some Afghans were less moved by the news that fewer civilians had been killed, and more concerned about the impact of armed groups on security, reflecting another of the report's findings.

The UN warned about the sanctioned use of local militias and their recruitment to the Afghan Local Police, particularly in the north and north-east of the country. The ALP is a localised militia-based force whose training was suspended by US forces in September owing to a dramatic rise in the number of inside attacks carried out by recruits.

According to the UN, the use of the ALP "unintentionally contributed to expand and solidify the power of armed groups" in the north.

"The Americans are paying too much heed to the Afghan Local Police, this what the Soviets did, it didn't work under them and it won't work for the foreign forces," said Wahid Ahmadi, a university student in Takhar city."In the northern provinces, where the government has little control, when there is an operation, no journalist or member of civil society is able to access or witness what happens."

He added that he believed the UN was not properly counting civilian deaths by Afghan and foreign forces.

The north saw some of the worst targeted assassinations by insurgents of government employees, a tactic which also spiked, the report said, resulting in a dramatic 700% increase in the number of civilian casualties attributed to this type of violence in 2012.

The report cited two particular attacks in northern Afghanistan that targeted government officials but resulted in the additional loss of life and injury of civilian bystanders.

By contrast the number of Afghan civilians killed by Nato-led and Afghan security forces dropped by nearly 40%. This figure may reduce anger at the alliance after it was chastised by President Hamid Karzai for carrying out an air strike in south-east Kunar province last week that killed 10 civilians.

The incident led Karzai to introduce a new decree, enforced on Monday, banning the Afghan military from requesting aerial support from Nato-led forces. The commander of international forces in Afghanistan, General Joseph Dunford, has agreed to the decree and maintained that the order will not prevent foreign troops from operating effectively.

The International Security Assistance Force in Kabul said it supported the UN figures. "Much work has already been done by Isaf in reviewing our tactical directives and operational procedures. We will aggressively work with the [Afghan forces] to enhance their civilian casualty mitigation efforts," Isaf spokeswoman in Kabul, Erin O. Stattel, said via email.

Unsurprisingly air strikes were the main cause of civilian casualties by foreign forces in 2012, but they killed 42% fewer civilians compared with 2011. Overall, air strikes in 2012 killed 126 civilians, including 51 children.

A spokesman for the Taliban dismissed the report, calling it "a vehicle of propaganda against" the group. "We don't accept this report, this is a biased report. You cannot find a single operation by the Taliban in which civilians have been killed … We are raising a Jihad for the sake and for the freedom of the Afghan people," Zabiullah Mujahed told the Guardian.

The UN also said it had identified a shift in the Taliban's public relations efforts.

The group's promotion of insider attacks on foreign troops and the pin-pointing of military targets, the report said, showed "a heightened awareness" by Taliban leadership of a need to publicly demonstrate that it wants to protect Afghan civilians, support the peace process and win "hearts and minds".