WASHINGTON - The head of the United Nations nuclear watchdog agency said Thursday that inspectors had made no progress in a yearlong effort to determine whether Iran had conducted research needed to build an atomic bomb.

Inspectors from the International Atomic Energy Agency are expected to meet Iranian officials in Tehran next week to seek a resumption of their inquiry on the possible military dimensions of Iran's nuclear program.

"We have intensified our dialogue with Iran this year, but no concrete results have been made yet," Yukiya Amano, head of the IAEA, told the nonpartisan Council on Foreign Relations.

The IAEA first reported in November 2011 that intelligence suggested Iran had conducted research that could help it develop a nuclear weapon. Iran maintains that its nuclear effort is for peaceful purposes, but it has refused to cooperate fully with IAEA inspectors.

The inspectors are seeking access to a suspected explosives testing facility at Parchin, south of Tehran. Satellite imagery indicates that Iran has tried to scrub evidence at the military base by demolishing buildings and removing soil that might hold traces of illicit nuclear work.

There's no evidence Iran has developed the capability to build a workable weapon.

"What we are asking in the negotiations is to have access to sites, information and people," Amano said.

The developments have sharpened a dispute between the White House and members of Congress who want to tighten economic sanctions against Tehran.

The Senate last week approved measures to blacklist companies or individuals doing business with energy, shipping and other industries that allegedly support Iran's nuclear program.

The Obama administration opposes new sanctions, saying they could undermine the international coalition that enforces sanctions already in place.