THE HAGUE, Netherlands — President Barack Obama hailed a “fundamental shift” in international efforts to fight nuclear terrorism as 35 countries pledged Tuesday to turn guidelines on nuclear security into national laws.

At the close of a two-day summit, the group also agreed to open up their security procedures to independent review, a further step toward creating an international legal framework to thwart nuclear terrorism.

The move is a joint initiative sponsored by host country the Netherlands, along with past summit hosts the United States and South Korea.

“I believe this is essential to the security of the entire world,” Obama said at a press conference wrapping up the summit.

He added that more still needs to be done: “Given the catastrophic consequences of even a single attack, we cannot be complacent.”

All 53 countries that participated in the Nuclear Security Summit in The Hague agreed Tuesday to keep looking for ways to ensure that nuclear material doesn’t fall into the hands of terrorists.

But the pledge to adopt nuclear guidelines into law and undergo external checks was endorsed by just 35 nations, including France, Britain, Canada, Japan, Israel. Notably absent from that agreement were Russia, China, India and Pakistan. North Korea and Iran didn’t even attend.

“We need to get the rest of the summit members to sign up to it, especially Russia, and we need to find a way to make this into permanent international law,” said Miles Pomper of the James Martin Center for Nonproliferation Studies.

Speaking to The Associated Press after the summit, Dutch Prime Minister Mark Rutte said the number who signed up was beyond his expectations and he expects more nations to sign up in coming days.

“This is a process of a big fleet of ships … — 53 countries, four international organizations,” he said. “We have agreed to a very ambitious communique and at the same time we have a part of this fleet which is even wanting to go further and is inviting the others to join them. “

Since Obama launched the summits in 2010, the number of countries that have enough material to build a nuclear weapon has fallen from 39 to 25.

This summit featured new reduction commitments, with Japan, Italy and Belgium pledging to reduce their stocks of highly enriched uranium and plutonium.

Other groups of nations pledged to step up efforts to prevent trafficking of nuclear material, boost maritime security and to develop low-enriched uranium for research reactors instead of the highly enriched, weapons-grade nuclear fuel currently widely used.

“I’ll close by reminding everyone that one of the achievements of my first summit in 2010 was Ukraine’s decision to remove all of its highly enriched uranium from its nuclear fuel sites,” Obama said. “Had that not happened, those dangerous nuclear materials would still be there now. And the difficult situation we’re dealing with in Ukraine today would involve yet another level of concern.”

Obama went on to conclude that “the more of this material we can secure, the safer all of our countries will be.”

The summit statement also reminded countries to consider the potential threat posed by cyberterrorism.

European Commission President Jose Manuel Barroso said the nuclear summit was crucial at a time when “international stability and security are being put into question in a very serious manner” — another reference to the Ukraine crisis.

“It is therefore all the more important to show that we are united in our commitment to a multilateral order based on peace and the unequivocal respect of the rule of law,” he said.

Michelle Cann of the Partnership for Global Security said Tuesday’s deal was significant because it marks “a change in the way security is done.”

Cann said the gains from reducing nuclear material will likely trail off in the future. In absence of a global treaty on nuclear security, the next phase in improving security will be for groups of like-minded countries to lead the way.

That will put peer pressure among nations, she said, with none wanting to be seen as the most lax at safeguarding their nuclear material.

Agreeing to external checks, conducted under the auspices of the International Atomic Energy Agency, is especially important, she added.

A final summit is scheduled for 2016 in the United States, but the process of securing nuclear material will not end there.

“Obama is thinking about a sustainable nuclear security system post-2016 so creating the architecture will be his homework,” said Chang-Hoon Shin, a research fellow at the Asan Institute for Policy Studies. “Sustainable, global nuclear security architecture is a long-term aim.”