Geneva: UN human rights investigators called on the United States on Friday to disclose whether there had been any plan to capture Osama bin Laden and if he was offered any "meaningful prospect of surrender and arrest."
Principles of engagement in such operations require the possibility of surrender, firing warning shots and if necessary wounding a suspect, rather than killing him, they said.
Failure to comply could amount to a "cold-blooded execution" but the overall situation must be taken into account, including whether US forces were under attack, said Martin Scheinin, UN special rapporteur on protecting human rights while countering terrorism.
UN has stressed that all counter-terrorism operations must respect international law.
"We are just saying the US government should answer questions concerning whether a meaningful prospect of surrender and arrest was given by the US, but perhaps not taken by Osama bin Laden," Scheinin told Reuters in a telephone interview.
Pakistani security officials have charged that US troops, after landing by helicopter at the Abbottabad compound, shot the unarmed al Qaeda leader in cold blood rather than in a firefight, as US officials first suggested.
It remained unclear whether the possibility of bin Laden's surrender had been built into the US assault on the al Qaeda leader's hideout in Pakistan on Monday, according to Scheinin.
"You design an operation so that there is a meaningful possibility of surrender and arrest even if you think the offer will be refused and you have to resort to lethal force," he said.
"It is the overall situation that governs when resorting to lethal force is permissible," Scheinin said.
Earlier, Scheinin and Christof Heyns, UN special rapporteur on extrajudicial, summary or arbitrary executions, said that in certain exceptional cases, deadly force may be used in "operations against terrorists."
"However, the norm should be that terrorists be dealt with as criminals, through legal processes of arrest, trial and judicially-decided punishment," the independent experts said in a joint statement.
"In respect of the recent use of deadly force against Osama bin Laden, the United States of America should disclose the supporting facts to allow an assessment in terms of international human rights law standards," they said.
"It will be particularly important to know if the planning of the mission allowed an effort to capture bin Laden."
Scheinin, a Finnish law professor who teaches in Florence, and Heyns, a South African human rights law professor, report to the UN Human Rights Council whose 47 members include the United States.
A US acknowledgment that bin Laden was unarmed when shot in the head in its operation at his hideout in Pakistan on Monday - as well as the sea burial of his body, a rare practice in Islam - have drawn some criticism in the Arab world and Europe, where some have warned of a backlash.
Al Qaeda confirmed bin Laden was dead on Friday, dispelling some of the fog around the killing of the "holy warrior," and vowed to mount more attacks on the West.
Navi Pillay, the top UN human rights official, also called this week for light to be shed on the killing, stressing that all counter-terrorism operations must respect international law.
"We've raised a question mark about what happened precisely, more details are needed at this point," her spokesman Rupert Colville told a briefing in Geneva on Friday.