Secretary General of Nato, Anders Fogh Ramussen, has tweeted a photograph of the flags at half-mast at NATO headquarters in Brussels to mark the 9/11 anniversary.
In a second tweet, he added: "Deepest sympathy to people of all our countries who suffered from terrorism. NATO's commitment to fighting terrorism remains unwavering."
Princes William and Harry cracked jokes with each other as they made multi-billion pound deals for charity on a City trading floor.
A man widowed in the September 11 attacks has returned to the top of the World Trade Center to honour the memory of the wife he lost there.