Two United Nations human rights watchdogs have asked the US to provide details about the operation that killed Osama bin Laden, in particular whether it ever included the possibility that he could be captured alive.

A series of questions have arisen about the potential legality of the mission after it emerged that four of the five people killed when US Navy Seals raided the house in Abbottabad, Pakistan, were unarmed, Bin Laden among them.

Pentagon officials initially talked of "a great deal of resistance" from inside the compound, but it was revealed that American forces only came under fire in the first few minutes of the operation.

In a statement released in Geneva, the UN's special rapporteur on extrajudicial, summary or arbitrary executions, Christof Heyns, and the special rapporteur on the promotion and protection of human rights and fundamental freedoms while countering terrorism, Martin Scheinin, said the US "should disclose the supporting facts to allow an assessment in terms of international human rights law standards".

They added: "For instance it will be particularly important to know if the planning of the mission allowed an effort to capture Bin Laden. It may well be that the questions that are being asked about the operation could be answered, but it is important to get this into the open."

Their call follows a demand by the UN's high commissioner for human rights, Navi Pillay, for "a full disclosure" of the facts connected to Bin Laden's death.

In their statement, Heyns and Scheinin acknowledged that such issues were difficult to gauge during anti-terrorism operations. "Acts of terrorism are the antithesis of human rights, in particular the right to life. In certain exceptional cases, use of deadly force may be permissible as a measure of last resort in accordance with international standards on the use of force, in order to protect life, including 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. Actions taken by states in combating terrorism, especially in high profile cases, set precedents for the way in which the right to life will be treated in future instances."

Both rapporteurs are law professors. Heyns specialises in human rights law at the University of Pretoria in South Africa and Scheinin is professor of public international law at the European University Institute in Florence.