A US soldier on active duty in Iraq has called US President Barack Obama an "impostor" in a statement in which he affirmed plans to join as plaintiff in a challenge to Obama's eligibility to be commander-in-chief.

The statement was publicised by California attorney Orly Taitz who, along with her Defend Our Freedom Foundation, is working on a series of legal cases seeking to uncover Obama's birth records and other documents that would reveal whether he meets the requirements of the US Constitution.