Arch-enemies Iran and Israel are "the closest" of eight hold-out nations to assuring the world they will never conduct a nuclear test explosion, a United Nations chief has said.
Lassina Zerbo, head of the UN's nuclear treaty organisation, said Iran and Israel ratifying pact 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 (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 UN General Assembly adopted the treaty in 1996 - the United States, China, Iran, Israel, Egypt, India, Pakistan and North Korea.
Mr Zerbo, speaking during a week-long conference marking the 20th anniversary of the treaty being opened for signing, said he did not expect immediate results on ratification but was 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".
He said implementing last summer's deal to rein in Iran's nuclear programme and confirmation from Israeli and international scientists that Tehran cannot produce nuclear weapons would mean "the biggest threat for Israel is gone and over".
Mr 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" towards the goal of nuclear non-proliferation and disarmament.
"Israel and Iran can make a huge difference for this treaty, and they have nothing to lose ... absolutely nothing," he 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.
Mr Zerbo said a nuclear test-free zone was an achievable step towards 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, Mr Zerbo said pressure would be put on the United States to follow suit.
US president Barack Obama wants to ratify the treaty, he said, but his hands were tied by the Republican-controlled Senate.
But Mr Zerbo said ratification by the three Middle East countries should convince conservative Republicans in the Senate to reconsider their opposition and support the treaty.
Looking at the current world situation and the other hold-outs, he said China would not ratify before the United States, India would not sign before China, and Pakistan would not ratify before India - which means US action was 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.
Mr Zerbo said the international community needed to change the way it engaged 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?'."
This, Mr Zerbo said, should have happened after North Korea's first test in 2006.