Allergy, not anguish in my Osama bin Laden photo: Hillary Clinton

An allergy and not anguish may explain why US secretary of state Hillary Clinton had her hand to her mouth while watching the commando operation to kill Osama bin Laden, she said on Thursday.

A photo of Clinton, President Barack Obama and other senior officials watching the operation live from the White House situation room has become one of the most striking images of the raid that killed the al-Qaeda leader.

The photo shows Clinton with her hand to her mouth in what looks like a gesture of anxiety over the outcome of the operation.

"Those were 38 of the most intense minutes. I have no idea what any of us were looking at that particular millisecond when the picture was taken," she said on Thursday when asked about the photo during a visit to Rome.

"I am somewhat sheepishly concerned that it was my preventing one of my early spring allergic coughs. So it may have no great meaning whatsoever," Clinton added.