Events will be held across the USA tomorrow to mark the 17th anniversary of the attacks.
The president and first lady Melania joined an observance at the september 11 memorial in a field near Shanksville, Pennsylvania, where one of the jetliners fell to the ground after 40 passengers and crew members realized hijackers had taken control and tried to storm the cockpit.
At the Pentagon, Vice President Mike Pence recalled the heroism of service members and civilians who repeatedly went back into the Pentagon to rescue survivors.
In a rousing pledge to the American people, Trump said he would do "everything in his power" to stop terrorists unleashing carnage in the US. "First responders are still dying and being ill", Chaundera Epps said, adding, "God bless America that's never, ever, stopped being great".
Americans were commemorating 9/11 with sombre tributes, volunteer projects and a new monument to victims on Tuesday (Wednesdasy NZ Time), after a year when two attacks demonstrated the enduring threat of terrorism in the nation's biggest city.
"We honour their sacrifices by pledging to never flinch in the face of evil and to do whatever it takes to keep America safe".
He said the 40 passengers and crew on United Flight 93 embodied heroism when they fought back against the hijackers.
"Departing Washington, D.C.to attend a Flight 93 September 11th Memorial Service in Shanksville, Pennsylvania with Melania". The passengers made a decision to take matters into their own hands.More news: Former President Obama delivers speech at University of IL
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He said the site marks the "moment when America fought back", and said the September 11 anniversary recalls the day "a band of courageous patriots turned the tide on our nation's enemies and joined the immortal ranks of American heroes".
The terror attacks were the deadliest in the U.S. history, killing 2,996 people.
In a field amid rolling hills, the 93-foot edifice was dedicated to the passengers on board who, after learning terrorists had moments before hit the World Trade Center and the Pentagon, chose to fight the terrorists who hijacked their jetliner.
Trump honored those killed 17 years ago at a rural field where the fourth airliner of the day crashed after those aboard realized what was happening and several passengers tried to storm the cockpit.
"We treasure their memory, we cherish their legacy and we ask God to forever bless the immortal heroes of flight 93".
Trump noted that almost 7,000 service members have died since 9/11, and - using terminology avoided by his predecessors - said they were "facing down the menace of radical Islamic terrorism".
Ahead of the ceremony, Trump was pictured doing a double fist pump at the airport after he disembarked Air Force One with Melania at his side.