Obama and Romney were invited earlier this summer to attend the Clinton Global Initiative (CGI), a summit official said, well before Clinton's Democratic National Convention speech last week where he gave a rousing endorsement of Obama and a detailed attack on the Republican presidential candidate.

Obama and Romney will address separate sessions on September 25, the final day of the three-day summit in New York City that brings together heads of state, business leaders, humanitarians and celebrities to make commitments to tackle the world's woes.