WASHINGTON -- President Donald Trump is presiding over his first 9/11 commemoration in office, a solemn and nonpartisan occasion, alongside first lady Melania Trump.

The Trumps observed a moment of silence at the White House on Monday in remembrance of the nearly 3,000 people who were killed when hijackers flew commercial airplanes into New York's World Trade Center, the Pentagon and a field near Shanksville, Pennsylvania.

The morning remembrance was scheduled for about the time the first plane struck one of the Twin Towers on the morning of Sept. 11, 2001.

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Mr. Trump and his wife also paid their respects at a Pentagon ceremony led by Defense Secretary Jim Mattis and Gen. Joseph Dunford, chairman of the Joint Chiefs of Staff.

"Our nation rallied together as one for while we had never asked for this fight, we are steadfastly comitted to seeing it through as president Trump has made it clear," said Mattis.

Mr. Trump called the Pentagon ceremony an "extraordinary" occasion, saying 16 years after the attacks, "we are one country and when we face hardship we emerge closer, stronger and more determined than ever."

"On that day not only did the world change but we all changed our eyes were opened to the depths of the evil we face. On that hour of darkness we came together with renewed purpose, our common bonds never felt so strong," he added.

American Airlines Flight 77 crashed into the military headquarters building and killed 189 people during the 2001 terrorist attacks.

Mr. Trump told families of victims of the attack that the entire nation grieved with them, saying, "The horror and anguish of that dark day were sealed into our nation's history forever."

"We can never erase your pain or bring back those you lost, we can honor their sacrifice to pledge our resolve to do whatever we can to keep our people safe," he urged.

He added, "The terrorists who attacked us thought they could incite fear and weaken our spirit, but america can not be intimidated and those who try will join the long list of vanquished enemies who dare test our mettle."

The observances come as Mr. Trump grapples with the death and destruction caused by two hurricanes in three weeks.

Shortly after, Vice President Mike Pence delivered remarks at an observance at the 9/11 memorial in Shanksville, Pennsylvania alongside Department of Interior Secretary Ryan Zinke.

"President Trump asked me to be here to pay a debt of honor to the heroes of flight 93, to all the fallen and their families, and the generation of heroes they inspired, and tell you that his heart is here in Pennsylvania, especially with all those who lost loved ones 16 years ago," said Pence.

An emotional Pence recalled being at the U.S. Capitol on 9/11 and learned the sequence of events that day, as a National Park Ranger informed Pence that flight 93 was en route to Washington when it crashed into the Pennsylvania field.

"I will always believe that I and many others at our nation's capitol, were able to go home that day, to hug our families because of the courage and selflessness of the heroes on flight 93, so for me it's personal."

The Vice President noted that on this day of remembrance, Americans must remember that the "threat remains and even now the menace of global terrorism brings violence across the wider world."

He added, "We've made great gains our resolve is stronger than ever and I promise you, together as one nation and one people, we will drive the cancer of terrorism from the face of the earth."

A native New Yorker, Mr. Trump has a mixed history with 9/11. He frequently uses the terrorist strikes to praise the city's response but also makes unsubstantiated claims about what he did and saw on that day.

Mr. Trump often lauds the bravery of New York police officers, firefighters and other emergency responders who rushed to the Twin Towers, in some cases knowing they probably wouldn't make it out alive, as an example of the resilience of the city where he made a name for himself.

But Mr. Trump has criticized President George W. Bush's handling of the attacks, accusing his fellow Republican of failing to keep Americans safe.

Mr. Trump has also made dubious claims about Sept. 11, particularly saying when talking about Muslims that "thousands of people were cheering" in Jersey City, New Jersey, across the Hudson River from lower Manhattan, as the towers collapsed. There is no evidence in news archives of mass celebrations there by Muslims.

Mr. Trump has also said he lost "hundreds of friends" in the attack and that he helped clear rubble afterward. Mr. Trump has not provided the names of those he knew who perished in the attack, but has mentioned knowing a Catholic priest who died while serving as a chaplain to the city's fire department.