UN envoy warns of US secret killings

Mr Alston said that he was aware of at least three such recent raids in the country's south and east and remarked that no-one is acknowledging responsibility for the killings.

He described one raid in January that had killed two Afghan brothers, which was conducted by Afghan troops and personnel from a US special forces base in Kandahar.

Afghan government officials have told him that the victims had no connection to the Taliban.

"It is absolutely unacceptable for heavily armed internationals accompanied by heavily armed Afghan forces to be wandering around conducting dangerous raids that, too often, result in killings without anyone taking responsibility for them," said Mr Alston, who is a special rapporteur to the UN human rights council on extrajudicial, summary and arbitrary executions.

He added that foreign intelligence agencies were operating with apparent "impunity" in certain provinces.

During a 12-day visit to Afghanistan, Mr Alston has met senior government ministers, the chief justice, the Afghan intelligence chief, international military commanders across the country, members of civil society and tribal elders.

"Based on my discussions, there is no reason to doubt that at least some of these units are led by personnel belonging to international intelligence services," he said.

"I am trying to encourage both the Americans and the Afghan government and others to take some of this seriously."

He said that there have also been raids in Nangarhar province, a known Taliban stronghold, where US special forces and other US-led units operate.

International military forces of all ranks that he had spoken to either claimed to be unaware of the raids or said that they could do nothing about it.

Mr Alston also voiced criticism of the tactics of both the Taliban and the Afghan police forces, pointing out that more than 500 civilians have been killed so far this year by Taliban militants, Afghan and US or NATO security forces and Afghan militiamen.

He accused the Afghan police forces of deep involvement in many unlawful killings.