Obama honours victims of 9/11
US President Barack Obama has honoured the nearly 3,000 people that were lost in the September 11 terrorist attacks.
In his weekly radio and internet address, Mr Obama said that while so much has changed in the years since 9/11 it is important to remember what has stayed the same.
"The core values that define us as Americans. The resilience that sustains us," he said on the eve of the 15th anniversary of one of the nation's darkest days.
He said the terrorists' goal is to frighten Americans into changing how they live, but "Americans will never give in to fear".
"We're still the America of heroes who ran into harm's way, of ordinary folks who took down the hijackers, of families who turned their pain into hope," Mr Obama said.
Nearly 3,000 people were killed in New York City, Pennsylvania and at the Pentagon when hijacked commercial airliners were slammed into all three locations in attacks that were planned and carried out by the al Qaida terrorist group.
Al Qaida leader Osama bin Laden was killed nearly a decade later by US forces during a May 2011 raid on his Pakistani hideout that Mr Obama authorised.
He noted in his address that the terrorist threat has evolved since September 11 "as we've seen so tragically from Boston to Chattanooga, from San Bernardino to Orlando," cities that suffered headline-grabbing, terrorist-linked, fatal shootings.
He pledged that the US will stay relentless against terrorism from al Qaida and the Islamic State group, which is spreading its mayhem across the Middle East and the West.
Mr Obama will mark Sunday's anniversary by observing a moment of silence in the privacy of the White House residence at 8:46 am EDT, when American Airlines Flight 11 crashed into the north tower of the World Trade Centre, before delivering remarks at a Pentagon memorial service.
Earlier New York Police Department marked 9/11 with a parade to honour its 23 officers who died at the World Trade Centre.
The parade through lower Manhattan also commemorated 99 officers who died of illnesses after participating in the rescue and recovery operation. Thirty-seven members of a port agency police department also died in the attack.
Police departments from around the US and Canada joined the procession.
Bagpipers led marchers around the southern tip of Manhattan, ending near a memorial wall engraved with the names of the fallen NYPD officers.
Police Commissioner William Bratton thanked other police departments for their support. Democratic mayor Bill de Blasio also attended.
The State Department is reminding US citizens about threats around the world and urging Americans to be vigilant about their personal security, ahead of the anniversary of the attacks.
The department said: "Current information suggests that terrorist groups continue to plan attacks in multiple regions" and that extremists "may use conventional or non-conventional weapons to target official government and private interests."
The US government said extremists remain interested in soft targets such as high-profile public events; hotels, clubs and restaurants; places of worship; schools; shopping malls and markets; public transportation systems; and events where Westerners gather in large numbers, including during holidays.
The department noted that US government facilities worldwide remain at a "heightened state of alert."