An Afghan police force funded and supported by the United States is getting away with serious abuses including rape and murder, Human Rights Watch (HRW) said in a report published on Monday.
The findings raise fresh questions about the Western exit strategy from Afghanistan and about handing full control of security to Afghan forces by the end of 2014, when all foreign combat troops are due to have left.
The 7,000-strong Afghan Local Police (ALP), set up last year and touted as key to the security handover, arms local people to protect their communities in areas where the Afghan army and regular police have limited reach.
They do not have law enforcement powers.
HRW found evidence of ALP abuses including killings, rapes and arbitrary detentions in three provinces -- Baghlan, Herat and Uruzgan -- out of seven where it conducted interviews.
It said such cases raised "serious concerns" about ALP vetting, recruitment and oversight and urged improvements including the establishment of a complaints body.
"Pressure to reduce international troop levels should not be at the expense of the rights of Afghans," said HRW's Asia director Brad Adams.
"Poor governance, corruption, human rights abuses and impunity for government-affiliated forces all are drivers of the insurgency and these issues need to be addressed if true stability is to come to Afghanistan."
The report said that the ALP "should be judged on whether it can bring security without violating the rights of the local communities it has been tasked to defend.
"If it becomes just another abusive militia, it will not only cause immense harm to local communities but risks undermining support for the central government and inflaming ethnic and political fault lines."
General David Petraeus, former commander of foreign forces in Afghanistan, in March called the ALP "arguably the most critical element in our effort to help Afghanistan develop the capability to secure itself".
A spokesman for the NATO-led International Security Assistance Force in Afghanistan, Lieutenant-Colonel Jimmie Cummings, said that it would work with the Afghan government to investigate the report's claims.
"The ALP programme is a critical component to bringing governance and security to the Afghan people at the local level," he said.
"Where relevant, we will endeavour to improve this programme and work diligently to correct these observations."
HRW also raised concerns about the role of Afghan government-backed militia groups controlled by local strongmen, which it linked to rapes, smuggling, extortion and targeted killings.
The militia are small groups loyal to warlords with roots in Afghanistan's bloody past, while the uniformed and salaried ALP was only set up last year.
"With patronage links to senior officials in the local security forces and the central government, these groups operate with impunity," the report said.
Afghan interior ministry spokesman Siddiq Siddiqui denied that the government had supported militia groups, insisting it was actually breaking them up after issuing a deadline for them to disband by October.
On the accusations of ALP abuses, he said that officials were looking into them.
"We take all accusations seriously," he said. "We'll investigate them and will take action if required."
In May, a separate report by Oxfam highlighted growing rights abuses by Afghan national police and troops, including killings and child sex abuse.
Afghanistan's army and police have grown quickly to over 300,000 and received billions of dollars of funding from the US in a bid to build them up ahead of the foreign combat force withdrawal in 2014.