The chief of the United Nations’ nuclear watchdog agency is headed to Tehran for meetings with President Hasan Rouhani and other senior officials in a sign of the group’s expanding role in forging a nuclear accord with Iran, despite concerns voiced by experts and U.S. lawmakers.

The Obama administration is positioning the International Atomic Energy Agency as a linchpin in its efforts to block Iran’s path to a nuclear weapon if a landmark deal is reached this month, according it a weight that worries some on Capitol Hill and in the Middle East...

...“I’m concerned. While I think the IAEA does some excellent work in general, the nature of what increasingly seems to be what they’re going to be called on to do…is greater than their present capacities,” said Sen. Bob Menendez (D., N.J.), a member of the Senate Foreign Relations Committee.

In particular, he said, the agency lacks the intelligence capacity to know whether Iran is cheating on any final deal, and would need more resources from the U.S. and five other global powers to fulfill an expanded role.

“This is a very large role that is being contemplated for a good agency, but one that is being stretched beyond its traditional limits,” Mr. Menendez said.

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