Published: Sep 10, 2010 6:46 PM EDT
Updated: Sep 10, 2010 3:45 PM EDT

NEW YORK (AP) - In the past, the anniversary of the Sept. 11 terrorist attacks was marked by somber reflection and a call to unity, devoid of politics. No more.

This year's commemoration of the attacks that killed nearly 3,000 people in New York, Washington and Shanksville, Pa., promises to be the most political and contentious ever, because of a proposed Islamic center and mosque near ground zero, and a Florida pastor's plan to burn the Quran - and the debate those issues have engendered over religious freedom.

As in other years, official ceremonies are planned at the three locations the terrorists struck. President Barack Obama will attend a  commemoration at the Pentagon, while Vice President Joe Biden will attend the ceremony at ground zero. First lady Michelle Obama and former first lady Laura Bush will travel to Shanksville to observe the anniversary there.

Obama told a White House news conference that Sept. 11 would be "an excellent time" for the country to reflect on the fact that there are millions of Muslims who are American citizens, that they also are fighting in U.S. uniforms in Afghanistan, and "we don't differentiate between 'them' and 'us.' It's just 'us."'

He said a plan by Terry Jones, the pastor of a small, independent church in Gainesville, Fla., to mark 9/11 by burning copies of the Quran must be taken seriously because it could cause "profound damage" to U.S. troops and interests around the world.

"You don't play games with that," Obama said, adding that as commander in chief he had an obligation to respond.

Jones said he called off his event under pressure from the White House and wouldn't follow through with burning the Muslim holy book if he was able to meet on Saturday with Imam Feisal Abdul Rauf, who is leading an effort to build the Islamic center and mosque near ground zero. Rauf said in a statement Friday he has no plans at this time to meet with Jones, although he is open to seeing anyone "seriously committed to pursuing peace."

Obama said he hopes Jones "prays on it and refrains from doing it," referring to the pastor as "the individual down in Florida."