International Atomic Energy Agency director general Yukiya Amano met US Senators on Wednesday to brief the body on the nuclear deal with Iran.

International Atomic Energy Agency director general Yukiya Amano met US Senators on Wednesday to brief the body on the nuclear deal with Iran.

Senate Foreign Relations Committee chairman Bob Corker said lawmakers left the briefing by the head of the UN nuclear watchdog agency "less assured" about the deal.

"The majority of members here left with far more questions than they had before the meeting took place," Corker told reporters.

"I can say from my perspective that it left me far less assured."

Amano traveled to Capitol Hill in a bid to assuage growing concern in Congress, where lawmakers will be voting in September on whether to approve the nuclear accord between Iran and world powers.

In particular, he was to address two confidential side deals the IAEA signed with Tehran.

Washington has described the side deals as "technical agreements," which are believed to include a deal about Iran's documentation of the alleged "previous military dimension (PMD)" of its nuclear program.

Iran in July granted the IAEA "managed access" to its military bases as part of the accord.

The IAEA agreement is aimed in part at resolving suspicions about Iran's military facility at Parchin, where US lawmakers claim Tehran conducted past nuclear work.

The IAEA chief told reporters he recognized the frustrations, but said he "explained that my legal obligation is to protect safeguards confidentiality."

Should Congress approve the joint comprehensive plan of action and it goes into effect, Amano stressed, "the nuclear activities of Iran will be reduced in size, and we'll have the most robust verification regime in Iran."