Services are being held across the United States and the world to mark 15 years since the 11 September attacks. <br/>Six moments of silence are being held in New York City, to mark the times four hijacked planes crashed and the two World Trade Center towers fell. Presidential candidates Donald Trump and Hillary Clinton are expected to attend a ceremony at the rebuilt site. <br/><br/>Ahead of the anniversary, President Obama said it was important to remember America's "core values".<br/><br/>President Obama held a moment of silence in the White House at 08:46 local time (12:46 GMT), the time the first plane hit the World Trade Center. He will later speak at an event at the Pentagon. <br/><br/>Neither presidential candidate will speak at the New York event.<br/><br/>Close to 3,000 people died when planes crashed into the towers, as well as the Pentagon in Washington DC and a field in Shanksville, Pennsylvania. Fifteen of the 19 attackers were Saudi nationals.