WASHINGTON (REUTERS) - President Barack Obama is leaning toward picking Caroline Kennedy to be the next United States (US) ambassador to Japan, a source familiar with the process said on Monday.

The 55-year-old daughter of former President John Kennedy would be the first female US ambassador to Japan.

She was one of the earliest backers of Obama in his first presidential campaign in 2008 and her endorsement was significant in helping him defeat Hillary Clinton in the Democratic presidential primary that year.

The author and lawyer serves on the board of several nonprofit organisations.

She would replace the current ambassador, Mr John Roos, the former chief executive officer of Silicon Valley-based law firm Wilson Sonsini Goodrich & Rosati.

White House spokesman Jay Carney declined to comment on whether Mr Obama had settled on Ms Kennedy for the post to represent the United States in Japan, one of America's top Asia allies.

"I have no personnel announcements to make," he said.

The Japan ambassadorship is an important diplomatic posting for the US at a time when Washington is keeping a careful eye on China as well as North Korea.