The head of the U.N. nuclear test ban treaty organization says arch-enemies Iran and Israel are "the closest" of the eight holdout nations to ratifying the treaty and assuring the world they will never conduct a nuclear test explosion.

Lassina Zerbo said this week that having Iran and Israel ratify together would "certainly" lead to Egypt's ratification, and pave the way for a nuclear test-free zone in the Middle East.

The Comprehensive Nuclear Test Ban Treaty, known as the CTBT, 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 that had nuclear power reactors or research reactors when the U.N. General Assembly adopted the treaty in 1996: the United States, China, Iran, Israel, Egypt, India, Pakistan and North Korea.

Zerbo, speaking during a week-long conference marking the 20th anniversary of the treaty being opened for signing, said he doesn't expect immediate results on ratification, but is hoping to visit both Iran and Israel and talk to their leaders because "I think that they're the ones who can unlock what is stopping the CTBT from moving."

In a briefing and an interview, he said that implementation of last summer's deal to rein in Iran's nuclear program — and confirmation from Israeli and international scientists that Tehran can't produce nuclear weapons — would mean "the biggest threat for Israel is gone and over."

Zerbo said the next step should then be to ratify the CTBT, which both Iran and Israel signed in 1996. He called this "a low-hanging fruit," toward the goal of nuclear nonproliferation and disarmament.

"Israel and Iran can make a huge difference for this treaty, and they have nothing to lose ... absolutely nothing," Zerbo said. "Both of them can take leadership and show carte blanche to the world to say we have together decided to ratify the CTBT."

He said ratification by Iran and Israel would help defuse tensions between the countries, build trust, and provide momentum — first for Egypt to ratify the CTBT and then to start negotiations for a nuclear test-free zone in the Middle East.

Zerbo said a nuclear test-free zone is an achievable step toward the much more difficult goal of establishing a nuclear-weapons-free-zone in the Middle East.

"You can't jump and get a weapon-free zone in the Middle East if the CTBT isn't ratified," he said.

Arab nations have been calling for a nuclear-free zone since the mid-1990s but efforts to hold a conference to discuss the possibility have failed. One key issue has been differences with Israel, which is widely believed to have an arsenal of hundreds of nuclear weapons but has avoided confirming or denying their existence.

But if Israel, Iran and Egypt ratify the CTBT, Zerbo said this will put pressure on the United States to ratify as well.

President Barack Obama wants to ratify the treaty, he said, but his hands are tied by the Republican-controlled Senate.

Zerbo said ratification by the three Mideast countries should convince conservative Republicans in the U.S. Senate to reconsider their opposition and support the treaty.

Looking at the current world situation and the other holdouts, Zerbo said, China won't ratify before the United States, India won't ratify before China, and Pakistan won't ratify before India — which means U.S. action is also crucial.

North Korea, the only country to test nuclear weapons in the 21st century, is least likely of the eight key countries to ratify the CTBT, he said.

Zerbo said the international community needs to change the way it engages with North Korea, which earlier this month said it exploded a hydrogen bomb in its fourth nuclear test, which has not been confirmed.

"What they need at this point in time is ... maybe a bit of respect and dignity in the dialogue we have with them," he said. "Instead of bang, bang on their head, maybe we have to come to sit with them around the table and say: 'Hey guys, if this is confirmed that it's the fourth test, we don't want this to happen again. How can we work?'"

Zerbo said this should have happened after North Korea's first test in 2006.