UNITED NATIONS: Israel should ratify the nuclear test ban treaty within five years — and Iran should also ratify but the timing is uncertain, the head of the UN organisation established to implement the treaty said Wednesday.

Lassina Serbo said in an interview with The Associated Press that Israel should be the next key country to ratify the treaty and he hopes it takes less than five years.

"I'm putting five years as the longest it should take now based on the positive sign that I'm seeing from Israel," said Serbo, who met Prime Minister Benjamin Netanyahu for the first time during a visit there in June.

He cited the impact of last year's Iran nuclear deal in the Middle East for "creating the confidence-building conditions in the region to help others to move forward."

Serbo said he has met Iran's Foreign Minister Mohammad Javad Sarif several times and the Iranians participate very actively in the test ban organisation.

"I think in Iran it's a matter of when, and the when will depend on the condition that will be right ... for them to consider the ratification," he said. "The only thing I say as head of the organisation is I hope the when is yesterday!"

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 UN General Assembly adopted it in 1996: the United States, China, Iran, Israel, Egypt, India, Pakistan and North Korea.

This year is the 20th anniversary of the treaty and Serbo was at UN headquarters for a panel organised by the General Assembly to mark Wednesday's International Day Against Nuclear Tests.

The Obama administration supports a worldwide ban on nuclear testing but hasn't ratified the treaty because it doesn't have the votes in the Republican-controlled Senate. China also reaffirmed its commitment to the treaty at Wednesday's meeting, but didn't say when it might ratify, Serbo said.

He said Pakistan has shown leadership recently, talking about a "bilateral moratorium with India" on nuclear testing. "They're waiting for a response from India," he said.

North Korea, the only nation that has tested nuclear weapons in the 21st century, was the only country of the eight key nations whose ratifications are needed to boycott Wednesday's General Assembly meeting. While its seat was empty, its underground explosions were criticised by Japan and many others.

Serbo said he advocates reopening a dialogue with North Korea "one way or another," stressing that nothing has stopped Pyongyang from carrying out nuclear tests and firing ballistic missiles.

"What I'm thinking is how can we get them to adhere to a moratorium on nuclear testing as an immediate and first step to denuclearisation," he said. "We have to find the means to do that."

Serbo said he thinks there's an opportunity to open a dialogue with North Korea, similar to the six-party talks with Iran that led to last year's nuclear deal, and possibly using the six-party talks aimed at negotiating the denuclearisation of the Korean peninsula which Pyongyang pulled out of in 2008.

He said one-on-one talks with countries close to North Korea can lead to wider international discussions because "it's the only way we can get them to stop."