US President Barack Obama has addressed the debate over the acquittal of the man who killed black Florida teenager Trayvon Martin, declaring that Mr Martin "could have been me, 35 years ago".

Florida neighbourhood watch volunteer George Zimmerman  was found not guilty of murder for shooting Mr Martin, 17, in a struggle in 2012.

Mr Obama expressed sympathy to the Martin family and praised them for the "incredible grace and dignity with which they've dealt with the entire situation".

He said the case was properly handled in the Florida court and acknowledged the relevance of the jury finding reasonable doubt in the prosecution's case.

He questioned "stand your ground" self-defence laws that have been adopted in 30 states.

Mr Obama, however, said Americans should understand the perspective of the black community, which has suffered a long history of racial discrimination.

"You know, when Trayvon Martin was first shot, I said that this could have been my son. Another way of saying that is Trayvon Martin could have been me, 35 years ago," he said somberly.

Mr Obama, 51, born in Hawaii to a black Kenyan father and white American mother, recalled his own encounters with racism and racial profiling.

"There are very few African-American men in this country who haven't had the experience of being followed when they were shopping in a department store. That includes me," he said.

He said he sometimes heard the clicks of car doors locking when he walked across the street in his younger days.

"There are very few African-Americans who haven't had the experience of getting on an elevator and a woman clutching her purse nervously and holding her breath until she had a chance to get off. That happens often," he said.

Citing the experiences of his teenage daughters, Mr Obama said younger generations have fewer issues with racism.

Still, he said, Americans need to do some "soul searching" on whether they harbour prejudice and should judge people not on the colour of their skin but on the content of their character.

"Each successive generation seems to be making progress in changing attitudes when it comes to race. It doesn't mean we're in a post-racial society. It doesn't mean that racism is eliminated ... We're becoming a more perfect union, not a perfect union, but a more perfect union," he said.

The Zimmerman verdict has produced a mixed reaction from Americans. A Reuters-Ipsos online poll found 34% agreed with the verdict, while 39% opposed it.

Beautiful tribute

Trayvon's parents, Sybrina Fulton and Tracy Martin, issued a statement on Friday praising Mr Obama and saying they were aware that their son's death and the jury's verdict had been deeply painful and difficult for many people.

"What touches people is that our son, Trayvon Benjamin Martin, could have been their son. President Obama sees himself in Trayvon and identifies with him. This is a beautiful tribute to our boy," they said.

Leaders of the black community also praised Obama.

"That our president has been profiled should encourage all Americans to think deeply about both the depth of this problem and how our country moves beyond it," said Benjamin Jealous, the president and chief executive officer of the NAACP, the nation's oldest civil rights group.