The United States on Sunday marked the 15th anniversary of the 9/11 attacks that claimed the lives of almost 3,000 people with President Barack Obama hailing the resilience and values that define and sustain Americans in his weekly address to the nation.

Obama also lauded the bravery of the survivors, the emergency personnel who responded following the attack on the twin towers of the World Trade Center and the Pentagon, and the efforts of several others who work day and night to keep the US safe from terror attacks.

"This weekend, we honour their memory once more. We stand with the survivors who still bear the scars of that day," Obama said in his weekly address.

"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," Obama said, adding that "we cannot give in to those who would divide us. We cannot react in ways that erode the fabric of our society."

"Because it's our diversity, our welcoming of all talent, our treating of everybody fairly-no matter their race, gender, ethnicity, or faith-that's part of what makes our country great. It's what makes us resilient. And if we stay true to those values, we'll uphold the legacy of those we've lost, and keep our nation strong and free," the US president further added.

Obama also said that a lot had changed in the last 15 years and also referred to the attacks in Boston, San Bernardino and Orlando. He vowed to keep fighting against Isis and al-Qaeda to keep the country safe.

"We delivered justice to (Al-Qaeda leader) Osama bin Laden. We've strengthened our homeland security. We've prevented attacks. We've saved lives... So in Afghanistan, Iraq, Syria and beyond, we'll stay relentless against terrorists like Al-Qaeda and ISIS. We will destroy them. And we'll keep doing everything in our power to protect our homeland," the president said.

Obama will also observe a moment of silence in the privacy of his White House residence at 8.46 a.m. local time, when American Airlines Flight 11 crashed into the World Trade Center's north tower, following which he will deliver remarks at a Pentagon memorial service.

Meanwhile, the New York Police Department honoured the 23 police officers who lost their lives in the attack and another 99 officers who died due to illnesses contracted due to the attack. They organising a parade that passed through lower Manhattan and ended near the memorial wall that had names of NYPD officers, who died in the attack, engraved on it.

Police departments from across the US and Canada as well as a team of bagpipers joined the NYPD parade and the memorial service.

A bike rally The Final Ride was also organised to mark the 15th anniversary of the 9/11 attacks in lower Manhattan on Aug. 21, 2016, Reuters reported.

Airplanes carrying passengers hijacked by 19 al-Qaeda terrorists crashed into the twin towers of the World Trade Centre in New York claiming the lives of almost 3,000 people. A third aircraft crashed into the Pentagon while the fourth one crashed in a field in Pennsylvania.