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 in Manhattan, 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 hours' long 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 Sept 11, 2001: It's the day she was born.

"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, 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, David, a Port Authority of New York and New Jersey police officer. 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 president, 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.

AP