While visiting Buenos Aires, President Obama said Wednesday, March 23, the U.S. will offer any assistance necessary to probe the Islamic State attacks in Brussels that killed at least 31 people and wounded some 270 others. (Reuters)

President Obama plans to convene a special session next week on global efforts to defeat the Islamic State as part of a major nuclear summit that he is leading in Washington, senior administration officials said.

Obama’s decision to add a session on the Islamic State to a summit that is focused on securing nuclear materials is a sign of the deep concern in the White House and worldwide after attacks in Paris; San Bernardino, Calif.; and Brussels.

Those strikes have exposed major shortcomings in the police and intelligence forces of many of America’s closest allies and raised new concerns about the nearly 6,000 “foreign fighters” from the West who are believed to have traveled to Iraq and Syria.

The White House is expected to announce plans for the additional counterterrorism-focused meetings with world leaders Friday, said senior administration officials who spoke on the condition of anonymity to preview the president’s plans.

The Nuclear Security Summit, which will bring as many as 50 world leaders to Washington, offers a rare forum for Obama to talk with his counterparts around the globe about the threat posed by the terrorist group. Planning for an extra session began in January after the Paris and San Bernardino attacks, senior administration officials said.

Brussels continues to morn the victims of bombing attacks that killed 31 people and wounded more than 270. But people are expressing a resolve not to let the fear of terrorism rule their lives. (Jason Aldag/Cléophée Demoustier for The Washington Post)

“This has been something we’ve been working on for quite a while,” one of the officials said.

Tuesday’s attacks in Brussels only added to the urgency of the effort.

All 50 nations represented at the nuclear summit have been sources of fighters for the Islamic State in Iraq, in Syria or elsewhere in the Middle East, the administration official said.

The nuclear summit — the fourth led by Obama since 2010 — has focused in previous years on securing vulnerable nuclear materials, breaking up black markets and intercepting illicitly trafficked materials. In addition to those topics, Obama and the world leaders will discuss ways to slow the flow of foreign fighters and money to the Islamic State in Iraq and Syria.

Breakdowns in intelligence-sharing between nations are also likely to be a major subject of discussion. Two of the siblings who blew themselves up in Tuesday’s attacks had entered Turkey illegally with the goal of eventually going to fight in Syria. The elder brother, Ibrahim el-Bakraoui, 29, was stopped by Turkish authorities and sent back to Europe, but Belgian officials said they didn’t track him as a potential terrorist threat. The younger brother, Khalid el-Bakraoui, 27, was also monitored by Turkey, but it’s not clear that Belgian authorities were aware of his movements.

After the Paris attacks, the Obama administration sent “foreign fighters surge teams” to Belgium and another European country to help local authorities improve intelligence-sharing among intelligence and law enforcement officials as well as across international borders. The teams also brought new technology to aid in the tracking of those fighters and organizational lessons that the United States learned in the nearly 15 years following the 9/11 attacks.

In Belgium, the foreign fighter teams’ efforts weren’t enough to stop Tuesday’s attacks. Still, a senior administration official said there is a “huge appetite” among European allies for help with the threat posed by foreign fighters. The summit could provide an opening to expand the teams and send them to other countries that are struggling to deal with the problem posed by foreign fighters.

The special session on the Islamic State is also expected to focus on the group’s ambitions to obtain a weapon of mass destruction. There is little indication that the Islamic State has access to the radioactive material needed to make a “dirty bomb.” “Syria and Iraq are free of highly enriched uranium and plutonium,” said a second senior administration official, who spoke on the condition of anonymity to discuss intelligence matters.

But the Islamic State has used crude chemical weapons, employing mustard gas and chlorine, in both Iraq and Syria. The terrorist group’s control over large areas of Iraq and Syria could give it access to experts and facilities that, over time, could allow it to develop more sophisticated and deadly chemical weapons.

Despite the growing fear caused by the recent attacks and by heated campaign trail rhetoric in the United States, senior White House officials maintain the administration’s strategy to defeat the Islamic State is working. They note that the group has lost 40 percent of the territory it once controlled in Iraq and 20 percent in Syria. The number of foreign fighters currently in Iraq and ­Syria has dropped to fewer than 25,000, down from 31,500 in 2014, according to U.S. intelligence estimates.

But as the terrorist group has come under pressure in Iraq and Syria, it appears to be morphing. Islamic State branches in Libya and Afghanistan have taken root and expanded in recent months. The group also has stepped up efforts to strike the United States and its Western allies.

“What distinguishes the threat today is that it is broader, more diffuse and less predictable than at any time since 9/11,” Lisa O. Monaco, a senior White House official focused on counterterrorism, said this month. “The primary example of this new type of terrorism is the cancer of” the Islamic State, she said.