The organisation has also written to the President of Yemen, asking whether there had been attempts by the Yemeni authorities to arrest these persons, and urging them to provide clarification about the cooperation and possible knowledge of the Yemeni government in this incident.

'If this was the deliberate killing of suspects in lieu of arrest, in circumstances in which they did not pose an immediate threat, the killings would be extra-judicial executions in violation of international human rights law,' Amnesty International said.

'The United States should issue a clear and unequivocal statement that it will not sanction extra-judicial executions in any circumstances, and that any US officials found to be involved in such actions will be brought to justice', the organisation added.

Amnesty International urges the United States Government to provide full clarification of the role played by US personnel in the death of the six men. Those killed include an alleged leading member of al-Qa'ida Qaed Salim Sinan al-Harethi.

Amnesty International recognises the obligation of the United States Government to protect its nationals. Nevertheless, any measure taken must be in accordance with US commitments under international human rights law. The prohibition against the arbitrary deprivation of life cannot be derogated from in any circumstances, even in a time of national emergency.