NEW YORK (Reuters) - President Barack Obama will warn Iran on Tuesday that the United States will "do what we must" to prevent it acquiring a nuclear weapon, and appeal to world leaders for a united front against attacks on U.S. diplomatic missions in Muslim countries.

In his address to the annual gathering of world leaders at the U.N. General Assembly, Obama will tell Iran there is still time for a diplomatic solution to the row over its nuclear program, but "that time is not unlimited." He will reiterate that Washington will never accept the idea that a nuclear-armed Iran could simply be "contained."

Obama, speaking exactly six weeks before the U.S. presidential election, will also urge unity in the face of a wave of Muslim outrage directed against the United States over a crude online video that denigrates the Prophet Mohammad.

"The attacks of the last two weeks are not simply an assault on America. They are also an assault on the very ideals upon which the United Nations was founded," Obama will say, according to advance excerpts from his speech released by the White House.

(Reporting by Matt Spetalnick; Editing by Kevin Liffey)