The Straits Times newspaper said it understood that the air marshals, who are equipped with stun guns and special bullets that fragment on impact to avoid puncturing the aircraft's fuselage, have been travelling on some flights since mid-August.

Singapore Airlines said it would not discuss the issue publicly because of security concerns.

A spokeswoman for the Singapore Government told the BBC that the introduction of air marshals was part of an on-going process to enhance current security measures, though she would not confirm the newspaper reports.