NYTimes.com -- Iran’s longtime ambassador to the International Atomic Energy Agency will leave in September, news agencies reported Wednesday, making him the third senior Iranian nuclear official to be replaced under the new administration of President Hassan Rouhani. The ambassador, Ali Asghar Soltanieh, frequently criticized the agency, the nuclear monitoring arm of the United Nations, as well as what he views as Western bias against Iran. Mr. Rouhani, who took office this month, has vowed to pursue a more flexible approach toward resolving the dispute over Iran’s nuclear program, which many countries suspect is aimed at giving it the capability to produce nuclear weapons, despite Iranian denials. Last Friday Mr. Rouhani replaced the head of Iran’s Atomic Energy Organization, which runs the country’s nuclear facilities. On Tuesday state news media signaled that negotiations over the nuclear program would now be handled by Mr. Rouhani’s foreign minister, Mohammad Javad Zarif, an American-educated diplomat.