Donald Trump has praised "the moment when America fought back" on United Flight 93, as he marked the 17th anniversary of the 9/11 attacks.

The president spoke with friends and families of victims of the plane, which crashed after crew and passengers stopped an attempted hijacking, saying in a speech: "America will never forget what your loved ones did for all of us."

"A piece of America’s heart is buried on these grounds," he said in Shanksville, Pennsylvania. "But in its place has grown a new resolve to live our lives with the same grace and courage as the heroes of Flight 93."

Across the country, cities like New York and Washington paid respects to the nearly 3,000 victims of the September 11th attacks, when terrorists flew planes into the Twin Towers in Manhattan, as well as the Pentagon.

Meanwhile, Mr Trump commemorated the 40 passengers and flight crew aboard the United Airlines flight headed to California that same day, celebrating their successful effort to thwart a terror attack on US soil.

A group of terrorists had hijacked the plane and were believed to be heading towards the nation’s capital, when several courageous passengers and members of the flight crew confronted them. The plane crashed into the Shanksville field that has since become a memorial ground.

"This field is now a monument to American defiance," Mr Trump said Tuesday, before providing "a message to the world".

"America will never, ever live in tyranny," he continued. "America’s future is not written by our enemies. America’s future is written by our heroes."

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The remarks were rather reserved for the president, who has been known to turn critical speeches into rambling discussions about the 2016 presidential elections, the looming federal investigation into Russian meddling or his former opponent, Hillary Clinton.

However, just before taking to the podium, a photograph of Mr Trump pumping his fists into the air quickly drew backlash from critics who felt it was inappropriate conduct for a president tasked with serving in the rare role of consoler-in-chief for the 17th anniversary of the attacks.

The president did spend his morning angrily tweeting about Russia and Ms Clinton, however, before writing in a post: “17 years since September 11th!”

He also tweeted about the "GREAT job" his private counsel Rudy Giuliani did while serving as mayor of New York City during the attacks 17 years ago. "Rudy is a TRUE WARRIOR!" Mr Trump wrote.

After his remarks, Mr Trump was provided a walking tour of the area in which Flight 93 had crashed, accompanied by his wife, First Lady Melania Trump.