US President Joe Biden has condemned white supremacists, the media, the internet and politics for spreading racist conspiracy theories as he mourned the killing of ten people in Buffalo, New York.

"What happened here is simple and straightforward: Terrorism, terrorism, domestic terrorism," he said.

Payton Gendron, an 18-year-old white teenager, is accused of opening fire with a semi-automatic rifle in a predominantly African-American neighborhood of Buffalo.

Authorities say hecarried out an act of "racially motivated violent extremism" o nSaturday at the Tops Friendly Market, when he shot 13 people.

Mr Gendron has been jailed without bail on a charge of first-degree murder. He has pleaded not guilty.

"White supremacy is a poison. It's a poison - it really is - running through our body politic," said Mr Biden, who spoke moments after meeting with families of the victims as well as first responders.

"We need to say as clearly and forcefully as we can that the ideology of white supremacy has no place in America. None."

US Attorney General Merrick Garland and other officials have called violence from white supremacists one of the biggest terrorism threats the United States faces, after related propaganda hit a record in 2020.

Investigators have said that they are looking into Mr Gendron's online postings, which include a 180-page manifesto he is believed to have authored that outlines the "Great Replacement" conspiracy theory that white people were intentionally being replaced by minorities through immigration in the US and elsewhere.

Mr Biden took aim at replacement theory, an idea with long historical roots in the US that is surging through some conservative political circles now.

"Now is the time for the people of all races, from every background to speak up as a majority of America and reject white supremacy"

Mr Biden did not lay out specific blame. However Fox News commentator Tucker Carlson was singled out by Democrats for allegedly stoking the theory in hundreds of episodes of his show.

Mr Carlson has called the Buffalo shooter "immoral" and "crazy."

"Hate and fear are being given too much oxygen by those who pretend to love America," Mr Biden said, blaming politics and profits.

"Now is the time for the people of all races, from every background to speak up as a majority of America and reject white supremacy," he said.

Mr Biden, joined by his wife, Jill, and a variety of New York political leaders stopped at a memorial set up under a tree to pay their respects near the supermarket where the gunfire rang out.

He reminded Americans that he ran for president to restore the soul of America, following predecessor Donald Trump's failure to denounce a deadly white supremacist rally in Charlottesville, Virginia, and took office weeks after a deadly attack on the US Capitol that included racially motivated groups.

But the Buffalo trip also showcased how little Mr Biden has achieved in stamping out a rise in white supremacist groups or curbing gun violence, with many Republican politicians blocking efforts to advance gun control measures and the country suffering a rash of mass shootings in recent months.

Mr Biden has asked Congress to require new background checks for gun buyers, and ban military-style "assault" weapons and large-capacity ammunition magazines.

However Democrats who largely support gun safety measures don't have enough votes to pass them.

Mr Biden told reporters before leaving Buffalo that he realises it will be hard to get legislation passed.

"It is going to be very difficult but I am not going to give up," he said on gun measures.

A top FBI official told Congress in November that the bureau was conducting around 2,700 investigations related to domestic violent extremism, and the Department of Justice said in January it was creating a new unit to counter domestic terrorism.