In his weekly address, Obama says the ''world's most deadly networks'' such as Islamic State must not be allowed to get their hands on weapons of mass destruction. Rough Cut (no reporter narration).
ROUGH CUT (NO REPORTER NARRATION) U.S. President Barack Obama urged world leaders to keep up efforts to safeguard vulnerable nuclear facilities to prevent terrorists like Islamic State from getting their hands on an atomic weapon or a radioactive "dirty bomb". Speaking in his weekly address from the Nuclear Security Summit in Washington, D.C., Obama said no group has succeeded in obtaining nuclear bomb materials but al Qaeda has long sought them, and Islamic State militants have already used chemical weapons in Syria and Iraq. Obama hosted more than 50 world leaders for his fourth and final summit focused on efforts to lock down atomic materials to guard against nuclear terrorism. A boycott by Russian President Vladimir Putin, unwilling to join in a U.S.-dominated gathering at a time of increased tensions between Washington and Moscow over Ukraine and Syria, may have contributed to summit results marked by mostly technical measures instead of policy breakthroughs. Despite significant strides by Obama in persuading dozens of countries to rid themselves of bomb-making materials or reduce and safeguard stockpiles, much of the world's plutonium and enriched uranium remain vulnerable to theft. Obama inaugurated the first Nuclear Security Summit nearly six years ago, after a 2009 speech in Prague laying out the goal of a world free of nuclear weapons. There is no guarantee that Obama's successor will keep the issue a high priority.