U.S. President Barack Obama said on Sunday that much has changed for Americans a decade after the attacks on Sep. 11, 2001, with the nation mired in war, recession and political divides, while "the wages of war are great."

"Ten years ago, America confronted one of our darkest nights," Obama said as he made a speech on the 10th anniversary of the 9/11 attacks at the Kennedy Center in Washington.

"In the decade since, much has changed for Americans," he said. "We've known war and recession, passionate debates and political divides. We can never get back the lives we lost on that day, or the Americans who made the ultimate sacrifice in the wars that followed."

Earlier the day, the president appeared at the ceremonies in New York City and Shanksville, Pennsylvania, and laid a wreath at the Pentagon Memorial later. He did not make speeches, but read passages from the Bible at the New York ceremony.

"Two million Americans have gone to war since 9/11. They have demonstrated that those who do us harm cannot hide from the reach of justice, anywhere in the world," Obama told his audience at the Kennedy Center, where a Concert for Hope was held to mark the anniversary.

"The sacrifices of these men and women, and of our military families, reminds us that the wages of war are great, that while their service to our nation is full of glory, war itself is never glorious," he said.

Ten years ago to the day, al-Qaida hijackers slammed three passenger planes into the World Trade Center in New York and the Pentagon, while a fourth hijacked plane crashed in a Shanksville field, killing nearly 3,000 people, making them the worst ever attacks on American soil.

The Bush administration launched two wars in Afghanistan and Iraq respectively in 2001 and 2003 in direct response to the attacks as it had announced a "global war on terror."