President Obama welcomes world leaders to Washington on Thursday for a two-day summit on nuclear security that aims to refocus global attention on an issue he has called a top priority but on which his administration has had limited success.

Seven years after he envisioned “a world without nuclear weapons” during a high-profile speech in Prague, Obama enters the last of four nuclear summits having proposed deep budget cuts next year on programs to stop nuclear proliferation while leaving intact military spending on a new generation of weapons.

Countries that have not given up stockpiles of nuclear material include the riskiest ones, such as Pakistan and India, which have fought four wars. And this week’s summit will have a glaringly empty chair: Russia, the world’s other nuclear superpower, has chosen not to attend amid tensions with the United States.

“The president has only accomplished a fraction of what he hoped to achieve,” said Joseph Cirincione, president of the Ploughshares Fund.

Obama has won a landmark agreement to limit Iran’s nuclear program, has persuaded about a quarter of the countries with loose nuclear materials to move them off their soil and signed with Russia a new START treaty that includes new weapons limits.

Japanese Prime Minister Shinzo Abe and President Obama shake hands in Manila on Nov. 19, 2015. The leaders of Japan and South Korea are among the heads of government coming to Washington for a summit on countering the threat of nuclear terrorism. (Susan Walsh/AP)

But after the exhausting negotiations with Iran, Cirincione said, “some of the steam has gone out” of Obama’s disarmament agenda.

White House officials defended the administration’s track record on nuclear security and disputed the suggestion that Russia’s absence is a significant setback. Deputy national security adviser Ben Rhodes noted Russia’s support of the nuclear pact with Iran, which hammered out between Iran, the United States and five other world powers, and said Moscow’s decision not to participate in the summit is a “missed opportunity for Russia above all.”

“All they’re doing is isolating themselves,” Rhodes said. He emphasized, however, that the United States maintains “ongoing cooperation and dialogue with them on issues related to nuclear security, and that’s important work that is ongoing.”

Obama launched the biennial summits with a gathering in Washington a year after his speech in Prague in April 2009. Since then, international summits have been held in Seoul and The Hague, and this week’s gathering is one of the president’s final chances to restore momentum to the project.

It comes at a fraught time.

North Korea alarmed Washington and its Asian neighbors with a provocative test of what it claimed was a hydrogen bomb in January, and mass-casualty terrorist attacks by the Islamic State and other groups have raised the specter of an even more deadly plot if such networks obtain nuclear material.

Aides said Obama will hold a separate bilateral meeting with Chinese President Xi Jinping, and a trilateral meeting with Japanese Prime Minister Shinzo Abe and South Korean President Park Geun-hye, to discuss North Korea and other matters. The White House has sought to convince Beijing to exert greater pressure on Pyongyang, and China supported additional economic sanctions imposed by the United Nations after North Korea’s most recent test.

On terrorism, Yukiya Amano, the head of the watchdog International Atomic Energy Agency, said in an interview Wednesday that the threat of groups such as the Islamic State obtaining nuclear materials is real.

In the past two decades, there have been about 2,700 reported incidents of missing radioactive materials, including some involving highly enriched uranium.

“We never know if we know everything,” Amano said. “This could be the tip of the iceberg. Some are illicit trafficking, very professional. Some people are trying to sell it. We have to think that the threat is real.”

Amano said one potential safeguard will come with an amendment to the Convention on the Physical Protection of Nuclear Material. The amendment requires countries to protect nuclear facilities and radioactive materials within their borders. Serbia and the Marshall Islands announced Wednesday that they would ratify it, and six more countries are needed for it to take effect. Amano said he expected more to sign on in the “near future.”

Meanwhile, Obama has announced a special session during the nuclear summit devoted to coordinating efforts to defeat the Islamic State, and the president also is expected to speak informally to world leaders on the sidelines of the meetings, White House aides said.

But even as new terrorist threats emerge, older concerns remain. Bruce Blair, a nuclear security expert at Princeton University, said the United States and Russia still have 1,700 weapons on launch-ready alert. Blair has been urging both governments to “relax their postures,” but he said two senior administration officials told him such action was not prudent.

“Dealing with Russia is the key to dealing with nuclear issues,” said former Democratic senator Sam Nunn (Ga.), who worked intensively on disarmament issues during his time on Capitol Hill and now is chief executive of the Nuclear Threat Initiative. “With both countries having the ability to destroy each other in a short time, the continuing posture, in my view, is very, very dangerous over a long period of time. We’ve been fortunate.”

Carol Morello contributed to this report.