The U.S. ambassador to the United Nations is set to meet with International Atomic Energy Agency (IAEA) officials for what she has described as a fact-finding mission as part of the U.S. administration's review of the 2015 nuclear deal with Iran.
President Donald Trump in April ordered a review of whether a suspension of sanctions on Iran related to the agreement between Tehran and world powers was in the U.S. national security interest.
The U.S. administration accuses Tehran of not abiding by the spirit of the accord, which placed curbs on Iran's nuclear program in exchange for sanctions relief.
Tehran, for its part, accuses Washington of violating the spirit of the deal by imposing new sanctions against the Islamic republic over its missile program.
A day before her meeting with IAEA officials in Vienna, U.S. Ambassador Nikki Haley told Reuters that Washington wanted to know if the UN atomic watchdog plans to inspect Iranian military sites to verify Tehran's compliance with the agreement.
"They have the authority to look at military sites now. They have the authority to look at any suspicious sites now, it's just are they doing it?" Haley said.
Tehran has rejected giving international inspectors access to Iranian military sites.
"Why would they say that if they had nothing to hide?" Haley asked. "Why wouldn't they let the IAEA go there?"
The IAEA said no public statements or news conference were planned on the talks.
The head of Iran's Atomic Energy Organization, Ali Akbar Salehi, said on August 22 that the Islamic republic could resume production of highly enriched uranium within five days if the nuclear deal was revoked.
"If we make the determination, we are able to resume 20 percent-enrichment in at most five days," Salehi told state media, adding that Iran's "biggest priority" was to maintain the nuclear accord but "not at any price."
Last week, President Hassan Rohani warned that Tehran could abandon the nuclear deal "within hours" if the United States continues with its policies of "sanctions and coercion."
The July 2015 nuclear deal gave Iran relief from international sanctions and imposed restrictions on its nuclear activities, including a cap on its uranium enrichment at under 5 percent.
High-level enrichment of uranium -- 20 percent or more -- would take Tehran close to the level needed for a nuclear weapon.
U.S. and UN watchdogs monitoring compliance with the agreement have found Iran has adhered to the deal.
However, the U.S. administration has frequently charged that Tehran violates the spirit of the agreement by continuing to test-launch ballistic missiles and rockets capable of carrying nuclear warheads.
The United States has imposed several rounds of sanctions over the missile and rocket launches, which Iran maintains are for self-defense and do not violate the deal.
Tehran has long said its nuclear program is for peaceful purposes.
Under U.S. law, the State Department must notify Congress every 90 days of Iran's compliance with the nuclear deal, which was negotiated under President Barack Obama. The next deadline is October.
The U.S. review of its policy toward Iran is also looking at Tehran's behavior in the Middle East, which Washington has said undermines U.S. interests.
U.S. State Department spokeswoman Heather Nauert said on August 15 that Washington believes the nuclear deal did not put an end to Tehran's other "destabilizing activities" in the region.