US President Barack Obama says a recent mass shooting at a historic black church in South Carolina shows that the "blight" of racism still exists in the United States.

On Wednesday, a 21-year-old white man opened fire at the Emanuel African Methodist Episcopal Church in Charleston, leaving nine African American worshippers dead.

Dylann Roof, the shooter, has posted his several pictures on his own hate-filled website pointing gun at camera and spitting on American flag.

Speaking at the annual US Conference of Mayors in San Francisco on Friday, Obama said the shooting incident shows the need for vigilance against racism.

"The apparent motivations of the shooter remind us that racism remains a blight that we have to combat together," Obama said.

He said that abundance of guns in America has helped hateful people to carry out such mass killings.

"You don't see murder on this kind of scale, with this kind of frequency, in any other advanced nation on earth," he said.

"Every country has violent, hateful or mentally unstable people. What's different is that not every country is awash with easily accessible guns," he said.

The church shooting has renewed perceptions around the world that Americans have too many guns and have yet to overcome racial tensions.

In Britain, Wednesday’s attack reinforced the view that America has an entrenched race and gun problem, according to The Associated Press.

In Australia and East Asia, where there are tight restrictions on firearm ownership and gun violence is almost unheard of, many were mystified by the tenacity of many Americans to own guns despite repeated mass shootings.

China’s official Xinhua News Agency said the massacre in South Carolina "mirrors the US government's inaction on rampant gun violence as well as the growing racial hatred in the country."

"Unless US President Barack Obama's government really reflects on his country's deep-rooted issues like racial discrimination and social inequality and takes concrete actions on gun control, such tragedy will hardly be prevented from happening again," Xinhua said in an editorial.

GJH/GJH