WASHINGTON (AFP) - Chelsea Manning, the US soldier serving a 35-year prison sentence for the biggest document leak in US, has a new job: writing unpaid for The Guardian newspaper's American edition.

A former Army intelligence analyst in Iraq, Manning was convicted in 2013 of having handed 700,000 classified documents to the anti-secrecy website WikiLeaks.

"Delighted to announce: Chelsea Manning joins @GuardianUS as a contributing opinion writer, writing on war, gender, freedom of information," Katharine Viner, editor-in-chief of the British newspaper's US edition, wrote on Twitter.

She indicated that Manning, formerly known as Bradley Manning, would not be paid.

The WikiLeaks source identifies as a woman and has sought gender reassignment treatment while serving her prison term.

Manning turned 27 in prison at Fort Leavenworth in Kansas, from where she will contribute her opinion pieces for the British newspaper's US website.