The US president stood with his hand to his chest, beside his wife Melania, and both closed their eyes.
They observed the moment at 8:46 am EDT in commemoration of the 18th anniversary of the attacks on the World Trade Centre in New York, NY, and the Pentagon in Washington, DC.
He was going to the Pentagon afterward to honour the nearly 3,000 people who died in the largest attack on American soil.
The President earlier tweeted a photo of himself and the First Lady, with the message "we will never forget" and the date of the attacks.
The world over there were mournful ceremonies and appeals to "never forget".
There were also bids at rising attention to the terror attacks' extended toll on responders.
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