A missile seen backdropped by a poster of the Iranian supreme leader Ayatollah Ali Khamenei.

The head of the U.N.’s nuclear test ban treaty organization says Iran should follow up on its historic nuclear deal with world powers by ratifying the treaty and assuring it will never conduct a nuclear test explosion.

Lassina Zerbo said in an interview Friday with The Associated Press that if Iran doesn’t ratify the treaty, “it will leave room for the doubt that people have put in this deal and the good intentions of Iran.”

Zerbo said Iran should have signed the Comprehensive Nuclear Test Ban Treaty, known as the CTBT, before negotiations started on the deal to rein in its nuclear program, in order to give assurances to critics that it has no intention to develop nuclear weapons – and that there is a religious prohibition, or “fatwa,” against possessing them, issued in 2013 by Iran’s supreme leader, Ayatollah Ali Khamenei.

Ratifying the treaty now, he said, will assure doubters in the U.S. and elsewhere that before or after the nuclear agreement ends in 15 years, Iran will never conduct any nuclear test explosions in a search to develop nuclear weapons.

Zerbo said ratifying the treaty will also give Iran a stronger position to say that it has complied with all arms control treaties and is part of the Nuclear Nonproliferation Treaty, so “what else do you need from us to show good faith with regard to our intention to only use nuclear energy for peaceful purposes?”

“I believe in what they say, and that’s why I’m telling them, ‘I trust you,’” he said of Iran. But he noted the saying “Trust, but verify” and added, “Why don’t you give us this assurance?”

The Comprehensive Nuclear Test Ban Treaty

The CTBT organization has 196 member states – 183 that have signed the treaty and 164 that have ratified it. But the treaty has not entered into force because it still needs ratification by eight countries involved in originally negotiating it: Iran, Israel, Egypt, India, Pakistan, North Korea, the United States and China.

If Iran ratifies the treaty, Zerbo said, it would create the conditions in the Middle East for easier ratifications, potentially by Egypt, and would be an essential element for the ultimate goal of creating a nuclear weapon-free zone in the region.

Monitoring

While waiting for the CTBT to enter into force, the organization has spent more than $1 billion on an international monitoring system that can detect a nuclear test by Iran or any other country, Zerbo said. It detected all three tests by North Korea.

The system uses four technologies – 170 seismic stations to pick up shock waves from any underground explosion, “hydro-acoustic” monitoring to pick up anything that happens underwater, 48 “infrasound” stations to detect low-frequency waves in the atmosphere that can travel long distances, and 80 “radionucleide” stations to sample the air for radiation, which Zerbo called “the smoking gun.”

The highly sophisticated monitoring systems can also detect earthquakes, tsunamis, air contamination, the movement of whales and the trajectory of space launches, as well as meteors including the ones in Russia in February 2013 and over Thailand on Sept. 7, he said.