WASHINGTON, Sept 22: At least 400 Americans have topped the one-billion-dollar level in net worth, Forbes magazine reports in its annual survey of the wealthiest people in the United States.

The latest survey, released on late Thursday, was the first time all members of the Forbes 400 were billionaires, the magazine said.

The collective net worth of the wealthiest Americans increased by $120 billion over the past year, to $1.25 trillion.

Microsoft founder Bill Gates kept his top spot on the Forbes list for the 13th straight year as his fortune grew to $53 billion.

Number two on the list was star investor Warren Buffett, who announced plans this year to give away 85 per cent of his fortune. He saw his net worth increase to $46 billion, Forbes said.

The top climber on the list was Sheldon Adelson, a self-made Las Vegas casino mogul who joined the top 10, listed third on the list with a net worth of $20.5 billion.

Fourth richest on the list was Larry Ellison, founder of business software giant Oracle with a net worth of $19.5 billion, followed by Paul Allen (16 billion).

Five members of the Walton family, the heir to the Wal-Mart fortune, were among the top 11 on the list. Jim Walton was the wealthiest at number six on the list with $15.7 billion, followed by his sister Christy (15.6 billion) and brother Robson Walton (15.6 billion).

Another Walton sibling, Alice Walton, was tied for ninth wealthiest (15.5 billion), and the matron of the family, Helen Walton, was ranked 11th (15.3 billion).

Personal computer mogul Michael Dell was tied for ninth on the list with an estimated worth of $15.5 billion.

Google's co-founders Sergey Brin and Larry Page were also prominent on the list. Brin was number 12 with $14.1 billion dollars and Page right behind with 14 billion, Forbes said. —AFP