Representatives from 56 nations and organizations will gather in Washington Thursday for the two-day Nuclear Security Summit. The conference, aimed at “strengthening the global nuclear security architecture,” is President Barack Obama’s fourth and final one.

“Thursday in Washington, I’ll welcome more than 50 world leaders to our fourth Nuclear Security Summit to advance a central pillar of our Prague Agenda: preventing terrorists from obtaining and using a nuclear weapon,” Obama wrote in an Washington Post op-ed published Wednesday. “We’ll review our progress, such as successfully ridding more than a dozen countries of highly enriched uranium and plutonium. Nations, including the United States, will make new commitments, and we’ll continue strengthening the international treaties and institutions that underpin nuclear security.”

The summit comes amid enduring concerns over the security of nuclear materials in high-risk facilities, such as those located in Pakistan, and fears that the Islamic State group, which has been stepping up its offensive in Europe, may find ways to develop radioactive “dirty bombs.”

Since the conclusion of the previous Nuclear Security Summit at The Hague in 2014, an intensification of attacks within Pakistan, mostly carried out by Taliban-affiliated groups, has triggered fresh concerns about the safety of its nuclear stockpile. Most recently, on Sunday, over 70 people were killed in a terrorist attack in Lahore.

The Asian nation, and its neighbor India — which it has fought four wars with — are among the countries that have not given up their nuclear stockpile, and refused to sign the multi-lateral Non-Proliferation Treaty (NPT).

“Nuclear safety is of obvious concern to both our countries [U.S. and Pakistan], and I expect that we will continue to discuss the obligations of being a responsible state with nuclear weapons in the coming year,” U.S. Secretary of State John Kerry said last month, after meeting Pakistani foreign affairs adviser Sartaj Aziz. “As you know, the United States of America once had 50,000 warheads pointing at another country or entity, the Soviet Union. And the Soviet Union had 50,000 pointing at us. And it took two presidents – President Gorbachev and President Reagan – in a meeting to decide that that didn’t make sense.”

Russia, however, would be conspicuous by its absence at the summit in Washington. The country, whose relations with the U.S. have rapidly deteriorated over the past few years, has refused to send representatives to the conference, citing a “lack of cooperation” by other countries over the agenda.

“Russia's decision to certainly not participate at a high-level we believe is a missed opportunity for Russia above all,” Ben Rhodes, the U.S. deputy national security advisor, said during a press briefing Wednesday. “Frankly, all they're doing is isolating themselves in not participating as they have in the past.”

The summit comes just two months after North Korea claimed it had tested a Hydrogen bomb. The announcement elicited immediate global condemnation, and the United Nations Security Council approved wide-ranging sanctions on the reclusive nation.

Over the next two days, Obama is also expected to hold talks with Asian leaders, including Chinese president Xi Jinping, over North Korea’s refusal to stop its missile tests.

“The destabilization in their neighborhood, these provocations out of North Korea, that is only counterproductive for China.  They do not benefit at all from seeing that type of behavior emanating from North Korea.  So we do believe that we have a shared interest in preventing destabilization on the Korean Peninsula and in promoting denuclearization,” Rhodes said.