Secretary of State Hillary Rodham Clinton is urging Iran to seize a chance to credibly address concern about its nuclear program.
Clinton says the Group of Eight leading industrialized democracies hope this weekend's talks in Istanbul create an environment for diplomatic breakthroughs with Tehran.
She says Iran must demonstrate it is serious about negotiations.
Talks are taking place amid increased speculation that Israel or the U.S. could take military action if Iran doesn't halt its uranium enrichment.
The U.S. and many other countries fear Iran is trying to develop nuclear weapons. Tehran insists the program is peaceful.
Clinton spoke after a two-day meeting of the G-8's top diplomats. Its members include Britain, Canada, France, Germany, Italy, Japan, Russia and the U.S.