KABUL, Afghanistan — Western secret services in Afghanistan are acting like South American “death squads,” a United Nations human rights expert has claimed.

A spokesman of the United Nations Human Rights Council, Professor Richard Alston, said the intelligence agencies and Afghan militias were targeting suspected insurgency leaders with “impunity.”

Their missions, he said, were “unaccountable to any international military authority.”

Although he refused to identify which intelligence services he was talking about, his comments follow criticism of the activities of CIA units, often by military personnel from other nations operating in Afghanistan.

It is alleged that ultra-secret operations are frequently run outside the normal chains of Nato and American military command by CIA units that answer directly to the Pentagon.

“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,” Mr. Alston said.

Among specific cases he investigated for contraventions of international and humanitarian law, Mr. Alston cited one raid in Kandahar Province in January 2008 in which two brothers were killed and which led to widespread local protest.

“The victims are widely acknowledged, even by well-informed government officials, to have had no connection to the Taliban, and the circumstances of their deaths are suspicious,” Mr. Alston said.

Spokesmen for both the American forces and the American embassy in Afghanistan declined to comment.