U.S. President Barack Obama is opening his fourth and final nuclear security summit in Washington on Thursday, acknowledging that the world remains imperiled by North Korea’s nuclear weapons development and the possibility that the terrorist group Islamic State could set off radioactive dirty bombs.

Ahead of the summit, Obama is meeting at the White House with South Korean President Park Geun-hye and Japanese Prime Minister Shinzo Abe to discuss the North Korean threat in the wake of Pyongyang’s January nuclear test and a long-range missile launch in February.  Later, Obama is meeting with Chinese President Xi Jinping.

Obama said in a Washington Post opinion article that “the international community must remain united in the face of North Korea’s continued provocations…The United States will continue working with allies and partners for the complete and verifiable denuclearization of the Korean peninsula in a peaceful manner.”

China plays key enforcement role

The U.S. views China, long an ally of North Korea, as key in enforcing United Nations sanctions against North Korea for its nuclear weapons development.

White House foreign policy aide Ben Rhodes said Wednesday, “We’ve seen China step up in many ways in terms of applying pressure.”

Obama said the U.S. would seek to renew efforts to keep Islamic State jihadists from obtaining nuclear materials to set off new terrorist attacks.

He said his three previous nuclear security summits have been successful in prodding more than a dozen countries to get rid of their supplies of highly enriched uranium and plutonium. He also said the U.S. and Russia are on track to cut their nuclear stockpiles to this lowest levels since the 1950’s.

Obama, however, said that Moscow and Washington, with 90 percent of the world’s nuclear weapons, “should negotiate to reduce our stockpiles further.” Obama said the two countries’ “massive Cold War nuclear arsenal is poorly suited to today’s threats.”

Russian President Vladimir Putin attends a meeting with German Foreign Minister Frank-Walter Steinmeier at the Kremlin in Moscow, Russia, March 23, 2016.

Russian President Vladimir Putin attends a meeting with German Foreign Minister Frank-Walter Steinmeier at the Kremlin in Moscow, Russia, March 23, 2016.

Russia’s absence

World leaders from 50 countries are attending the summit, but not Russian President Vladimir Putin, who has had a testy relationship with Obama, particularly after Moscow annexed Ukraine’s Crimean peninsula two years ago and Obama responded by leading a Western effort to impose economic sanctions on Russia that remain in place and have contributed to its recession.

Kremlin spokesman Dmitry Peskov said Russia was skipping the summit because of a “shortage of mutual cooperation” in setting the agenda.

In a full page ad in the Post, a group of about 100 corporate and former government officials from around the world called on those attending the summit to accelerate efforts to set “global standards, accountability and best practices for securing all nuclear materials.” They said the U.S. and Russia had a “special responsibility” to lead the effort.

New initiative

As the summit opens, Britain is expected to announce that it will team up with the U.S. and European partners to exchange nuclear waste for material to be used to fight cancer.

The plan calls for London to send waste from its nuclear facilities in Scotland for processing in U.S. reactors, while the U.S. will send uranium for use in reactors controlled by the European Atomic Energy Community.

The White House says that while it is impossible to quantify the likelihood of a nuclear attack by extremist groups, there are 2,000 metric tons of highly enriched uranium and separated plutonium with civilian and military programs around the world.

“We know that terrorist organizations have the desire to get access to these raw materials and their desire to have a nuclear device,” Rhodes said.

Belgian soldiers stand guard next to one of the memorials to the victims of the recent Brussels attacks, at the Place de la Bourse in Brussels, March, 27, 2016.

Belgian soldiers stand guard next to one of the memorials to the victims of the recent Brussels attacks, at the Place de la Bourse in Brussels, March, 27, 2016.

Terror threat

The Brussels terrorist attacks on March 22 signaled again how dangerous and far-reaching groups like Islamic State have become. Since the attacks, Belgium has put armed guards at its nuclear facilities.

Leaders are especially concerned about the security of nuclear materials and facilities in countries such as nuclear-armed Pakistan, where a terrorist attack in Lahore on Easter Sunday killed more than 70 people.

While progress has been made since the first summit in 2010, “the overall objective of securing the most vulnerable nuclear materials in four years…I don’t think has been achieved,” said Sharon Squassoni with the Center for Strategic and International Studies. “There is still material out there and the material that we’re talking about is highly enriched uranium.”

Experts say security gaps remain for several reasons: there still is no international framework to monitor nuclear materials; some countries are unwilling to open up supplies intended for commercial use, and some militaries have been unable to agree on how to deal with their nuclear material.