A flower and US flag rest next to names at the south reflecting pool at the National September 11 Memorial and Museum as the Tribute in Light shines over lower Manhattan on the 16th anniversary of the 9/11 attacks in New York City, US, September 11, 2017. [Photo/Agencies]

NEW YORK — Americans commemorated 9/11 on Monday with tear-streaked tributes, a presidential warning to terrorists and appeals from victims' relatives for unity and hope 16 years after the deadliest terrorist attack on US soil.

Looking out at the solemn crowd at ground zero, Debra Epps said she views every day as time to do something to ensure that her brother, Christopher Epps, and thousands of others didn't die in vain.

"What I can say today is that I don't live my life in complacency," she said. "I stand in solidarity that this world will make a change for the better."

Thousands of family members, survivors, rescuers and others gathered for the hourslong reading of victims' names at the World Trade Center, while US President Donald Trump spoke at the Pentagon and Vice-President Mike Pence addressed an observance at the Flight 93 National Memorial near Shanksville, Pennsylvania.

Elsewhere, thousands of Americans marked the anniversary with service projects. Volunteer Hillary O'Neill, 16, had her own connection to 9/11: It's her birthdate.

"I always feel a sense of responsibility to give back on the day," O'Neill, of Norwalk, Connecticut, said as she packed up meals in New York City for needy local people and hurricane victims in Texas and Florida.

Nearly 3,000 people were killed when planes hijacked by terrorists hit the trade center, the Pentagon and a field near Shanksville on Sept 11, 2001, hurling America into a new consciousness of the threat of global terrorism.

Reflecting on a tragedy that still feels immediate to them, victims' relatives thanked first responders and the military, worried for people affected by Hurricane Irma as it continued its destructive path as a tropical storm and pleaded for a return to the sense of cohesiveness that followed the attacks.

"Our country came together that day. And it did not matter what color you were or where you were from," said a tearful Magaly Lemagne, who lost her brother, Port Authority of New York and New Jersey police officer David Lemagne. She implored people to stop for a moment and remember all the people who gave their lives that day.

"Maybe then we can put away our disagreements and become one country again," she said.

Trump, a native New Yorker observing the anniversary for the first time as the country's leader, assured victims' families that "our entire nation grieves with you" and issued stern words to extremists.

"America cannot be intimidated, and those who try will join a long list of vanquished enemies who dared test our mettle," the Republican president said as he spoke at the Pentagon after observing a moment of silence at the White House.

When America is united, "no force on earth can break us apart," he said.

A woman pauses at the south reflecting pool at the National September 11 Memorial and Museum in lower Manhattan on the 16th anniversary of the 9/11 attacks in New York City, US, September 11, 2017. [Photo/Agencies]

At the Flight 93 National Memorial, Pence said the passengers who revolted against hijackers might well have saved his own life.

The Republican vice-president was a member of Congress on 9/11, and the Capitol was a possible target of the terrorist piloting Flight 93. Instead, it crashed near Shanksville after the passengers took action. Thirty-three passengers and seven crew members were killed.

At the United Nations, some ambassadors noted the anniversary as the Security Council unanimously approved new sanctions against the Democratic People's Republic of Korea in response to its Sept 3 test of what Pyongyang said was a hydrogen bomb.

"We will never forget the victims of Sept 11, and we will never forget the lesson that those who have evil intentions must be confronted," US Ambassador Nikki Haley said.

At dusk, the annual "Tribute in Light" art installation beamed two giant towers of light into the lower Manhattan skyline as a visual memorial to those who perished in the terror attack.

The ceremony on the National September 11 Memorial & Museum plaza in New York strives to be apolitical, allowing politicians to attend but not to speak. Yet last year's 15th anniversary ceremony became entangled in the presidential campaign when Democratic nominee Hillary Clinton left abruptly, stumbled into a van and ultimately revealed she'd been diagnosed days earlier with pneumonia.

This year, the focus remained on the names read out beneath the waterfall pools and lines of trees.

While many Americans may no longer interrupt their days to observe the 9/11 anniversary, the ceremony remains a touchstone for many victims' families and friends.

"I'll come every year for the rest of my life," said Rob Fazio, who lost his father, Ronald Fazio. "It's where I get my strength."

After 15 years of anniversaries, the reading of names, moments of silence and tolling bells have become rituals, but each ceremony takes on personal touches. Name-readers Monday gave updates on family graduations and marriages and remembered loved ones' flair for surfing or drawing on coffee-shop napkins.

A few never even got to know the relatives they lost on Sept 11, 2001.

"I wish more than anything that I could have met you," Ruth Daly said after reading names in remembrance of her slain grandmother and namesake, Ruth Lapin.

Delaney Colaio read names in honor of the three relatives she lost: her father, Mark Joseph Colaio, and her uncles, Stephen Colaio and Thomas Pedicini. Just a toddler on 9/11, she is now making a documentary about the children who lost parents in the attacks.

"I stand here as a reminder to the other families of 9/11 and to the world," she said, "that no matter how dark moments of life can get, there is light ahead if you just choose hope."

AP

The Tribute in Light installation is illuminated over lower Manhattan as seen from Brooklyn marking the 16th anniversary of the 9/11 attacks in New York City, US, September 11, 2017. [Photo/Agencies]

An origami crane rests over a name at the south reflecting pool at the National September 11 Memorial and Museum in lower Manhattan on the 16th anniversary of the 9/11 attacks in New York City, US, September 11, 2017. [Photo/Agencies]

Flowers and US flags rest next to names at the south reflecting pool at the National September 11 Memorial and Museum as the Tribute in Light shines over lower Manhattan on the 16th anniversary of the 9/11 attacks in New York City, US, September 11, 2017. [Photo/Agencies]