It said that Mills, 68, died a few hours after being taken ill but no further details were given.
A presidential aide, who asked not to be named, said the president had complained of pains on Monday evening and died early on Tuesday afternoon when his condition worsened.
John Dramani Mahama, his vice-president, has been sworn in as his replacement in a ceremony in the capital.
Mr Atta Mills had ruled the West African country since 2009.
May his soul rest in Eternal peace...AMEN!!.