"Fifteen years ago, a September day that began like any other became one of the darkest in our nation's history," U.S. President Barack Obama said Saturday in his weekly address.

Sunday marks the 15th anniversary of the September 11, 2001, attacks on the United States targeting the World Trade Center in New York, the Pentagon near Washington, D.C., and bringing down a plane in rural Pennsylvania.

The president said nearly "3,000 innocent lives were lost" that September day "from all walks of life, all races and religions, all colors and creeds, from across America and around the world."

Obama said a lot has changed over the past 15 years, but "it's also important to remember what has not changed - the core values that define us as Americans. The resilience that sustains us… terrorists will never be able to defeat the United States."

FILE - A girl is seen at the National September 1 FILE - A girl is seen at the National September 11 Memorial and Museum in New York City, Sept. 8, 2013. FILE - A girl is seen at the National September 11 Memorial and Museum in New York City, Sept. 8, 2013.



On Sunday - the annual 9/11 National Day of Service and Remembrance - the president will observe a moment of silence at the White House. Later Sunday, Obama will speak at a ceremony at the Pentagon honoring those killed in the attacks.

Near Shanksville in western Pennsylvania, the Flight 93 National Memorial stands in memory of the passengers and crew members who carried out a sustained assault against the hijackers for control of the plane 15 years ago.

A September 11 Museum has been erected on the New York site where the World Trade Center once stood, housing artifacts and photographs connected to the attack.

At the Pentagon, the 184 people who died there on September 11, 2001 are honored with 184 benches over pools of water.