Leading his first commemoration of the solemn 9/11 anniversary, President Donald Trump said on Monday that 'the living, breathing soul of America wept with grief' for each of the nearly 3,000 lives that were lost on that day 16 years ago.

Addressing an audience at the Pentagon, one of three sites attacked on September 11, 2001, Trump used the anniversary to sternly warn terrorists that 'America cannot be intimidated'.

He said those who try are destined to join a long list of vanquished enemies 'who dared to test our mettle'.

Trump and first lady Melania Trump observed a moment of silence at the White House on Monday at the exact moment that a hijacked airplane was slammed into the World Trade Centre.

The Trumps bowed their heads and placed their hands over their hearts as 'Taps' rang out across the South Lawn.

They were surrounded by White House aides and other administration officials in what has become an annual day of remembrance.

Nearly 3,000 people were killed when hijackers flew commercial airplanes into New York's World Trade Centre, the Pentagon and a field near Shanksville, Pennsylvania.

Trump, a native New Yorker who was in the city on 9/11, said the attack was worse than the Japanese assault on Pearl Harbor during World War II because it targeted civilians. He vowed that it would never be repeated.

"The terrorists who attacked us thought they could incite fear and weaken our spirit," Trump said later at the Pentagon, where he was joined by Defence Secretary Jim Mattis and General Joseph Dunford, chairman of the Joint Chiefs of Staff.

"But America cannot be intimidated and those who try will join a long list of vanquished enemies who dared test our mettle."

He said that when America is united, 'no force on earth can break us apart'.

Trump also offered words of comfort the many whose loved ones perished in the attacks. -- PTI