A UN expert on Tuesday urged Washington to clarify its rules on hunting Taliban and other suspects amid a "dramatic increase" in the use of drone attacks.

The US government has carried out targeted killings in countries including Afghanistan, Pakistan, Somalia and Yemen inraids and airstrikes and used unmanned drones, a report by investigator Christof Heyns said.

"The Special Rapporteur reiterates his predecessor's recommendation that the government specify the bases for decisions to kill rather than capture 'human targets' and whether the state in which the killing takes place has given consent," said Heyns.

In the document Heyns, special rapporteur on extrajudicial and summary executions, provides an update on steps taken by the United States to implement the recommendations of a 2009 report.

"No information has been made available on substantial changes to procedures ... to ensure that strikes targeting Taliban fighters were based on reliable information and did not cause unnecessary suffering and damage to the civilian population," he said.

"The Special Rapporteur again requests the government to clarify the rules that it considers to cover targeted killings," the report adds.

About 300 drone strikes have been carried out in Pakistan since June 2004, according to the document.

It cited figures from the non-governmental Pakistan Human Rights Commission which said that US strikes were responsible for at least 957 deaths there in 2010.

"Although figures vary widely with regard to drone attack estimates, all studies concur on one important point: there has been a dramatic increase in their use over the past three years," said the report.

"While these attacks are directed at individuals believed to be leaders or active members of al-Qaeda or the Taliban, in the context of armed conflict, in other instances civilians have allegedly also perished in the attacks."

Heynes also said Washington should specify the safeguards it has in place to ensure in advance that targeted killings comply with international law.

"The Special Rapporteur is seriously concerned that the practice of targeted killing could set a dangerous precedent, in that any Government could, under the cover of counter-terrorism imperatives, decide to target and kill an individual on the territory of any state if it considers that said individual constitutes a threat," the report said.