The U.S. and the European Union have said Iran’s cooperation with the U.N. in addressing evidence that Tehran conducted studies in the past on the development of atomic weapons is crucial to reaching a broader accord on the future of the Iranian nuclear program.

But Yukiya Amano, director general of the International Atomic Energy Agency, the U.N.’s nuclear watchdog, said Friday there has been almost no progress in resolving the outstanding allegations of weapons development, despite a year of negotiations with Iranian President Hasan Rouhani ’s government.

The IAEA and Tehran last year outlined 12 areas of concern that needed to be addressed before the agency could close its file on Iran, which has been the target of four rounds of U.N. sanctions. Mr. Amano said there has been significant progress in only one of these areas.

“What is needed now is concrete action,” Mr. Amano said during a speech in Washington. “Progress is limited.”

The IAEA is seeking access to leading Iranian nuclear scientists who are believed to have been involved in nuclear-weapons research. 

The agency also is demanding access to Iranian research and military sites. Among them is a military base south of Tehran, called Parchin, where the IAEA and the U.S. believe atomic-weapons development occurred. 

Iran so far has denied Mr. Amano’s investigators access to the scientists or the sites, despite Tehran’s insistence that its nuclear program is solely for peaceful purposes. Iran also has said that the IAEA’s evidence of weapons work is based on fabricated intelligence.

Mr. Amano said the IAEA’s evidence is based on “broadly credible information.” He also said there is a possibility that Iran has continued to conduct nuclear-weapons research, even during the negotiations with the West. 

An intelligence estimate released by the George W. Bush administration in 2007 concluded that Tehran’s weapons research likely ended in 2003.

The U.S. and other powers are intensifying negotiations with Iran in an effort to meet the Nov. 24 deadline. The Obama administration is part of a diplomatic bloc, called the P5+1, which is made up of the five permanent members of the U.N. Security Council plus Germany.

The State Department announced Friday that Secretary of State John Kerry would meet with his Iranian counterpart, Javad Zarif, in Oman on Nov. 9-10 in a bid to support the nuclear negotiations. The final talks will then shift to Vienna.

U.S. officials have voiced guarded optimism that a deal could be reached by the November deadline. Absent that, they have suggested the diplomatic process could be extended.

Mr. Amano met Thursday with Mr. Kerry and spoke with White House National Security Adviser Susan Rice Friday.

State Department officials didn’t respond to questions about how Mr. Amano’s assessment might complicate the Vienna talks. But A number of the U.S.’s closest Middle East allies, particularly Israel and Saudi Arabia, have voiced concerns in recent weeks that the U.S. and P5+1 have softened their negotiating position with Iran.

Prince Turki al-Faisal, a leading member of the Saudi royal family, said his country could also seek to develop the technologies used in building nuclear weapons if Iran was allowed to maintain them as part of a final agreement. 

Riyadh will “also seek to have the same terms in developing our nuclear energy,” the Saudi royal said during a speech in Washington this week.