Staying in Washington, US President Barack Obama has led a national moment of silence at the White House for the victims of Saturday's shooting in the southwest state of Arizona.

The moment of silence was also observed at the US Capitol building and on the trading floor of the New York Stock Exchange.

On a frigid Washington morning, President Barack Obama and First Lady Michelle Obama walked out of the White House to the sounding of a bell at 11am local time.

Wearing overcoats, they stood next to each other on the South Lawn, each with their hands clasped, heads bowed and eyes closed.

More than 300 White House staff members, from senior advisers to members of the kitchen staff and maintenance crews, joined the Obamas.

The moment also was marked on Capitol Hill, where a flag flew at half mast.

In total, 19 people were shot in Saturday's attack in Tucson, Arizona.

Democratic congresswoman Gabrielle Giffords was shot point-blank in the head, and she remains in intensive care.

Among the six people killed were Arizona's chief federal judge, a 9-year-old girl, and one of Giffords' aides.

Prosecutors have charged 22-year-old Jared Loughner with one count of attempted assassination of a member of Congress, two counts of killing an employee of the federal government and two counts of attempting to kill a federal employee.

More charges are expected.