WASHINGTON (Reuters) - Just as fears of nuclear terrorism are rising, U.S. President Barack Obama’s drive to lock down vulnerable atomic materials worldwide seems to have lost momentum and could slow further. With less than 10 months left in office to follow through on one of his signature foreign policy initiatives, Obama will convene leaders from more than 50 countries in Washington this week for his fourth and final Nuclear Security Summit, a high-level diplomatic process that started and will end on his watch. A boycott by Russian President Vladimir Putin, apparently unwilling to join in a U.S.-dominated gathering at a time of increased tensions between Washington and Moscow, adds to doubts that the meeting will yield major results. The recent deadly militant attacks in Brussels have fueled concern that Islamic State could eventually target nuclear plants and develop radioactive “dirty bombs,” a topic that may well be uppermost in leaders’ minds as they meet. Despite significant progress 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 remains vulnerable to theft. The absence of Russia, one of the biggest atomic powers, could detract from decisions reached in Washington this week. Obama, in an opinion piece in The Washington Post, said, “Our massive Cold War nuclear arsenal is poorly suited to today’s threats. The United States and Russia - which together hold more than 90 percent of the world’s nuclear weapons - should negotiate to reduce our stockpiles further.” Kremlin spokesman Dmitry Peskov said on Wednesday Russia was skipping the summit because of a “shortage of mutual cooperation” in working out the agenda. While noting that Moscow had continued joint work on nuclear security, White House spokesman Josh Earnest said Russia was going to “miss out on an opportunity” and that its no-show  illustrated the “degree to which Russia is isolated.” Russia has chafed over U.S.-led sanctions over the Ukraine conflict. Efforts to make the world safer have also been complicated by North Korea’s nuclear weapons advance and Pakistan’s move toward smaller, tactical nuclear weapons, which Washington fears may further destabilize an already volatile region. All of this weighs on Obama’s agenda as he prepares to host world leaders on Thursday and Friday. He inaugurated the event nearly six years ago, after using a landmark speech in Prague in 2009 to lay out the goal of ridding the world of nuclear weapons as a central theme of his presidency. There is no guarantee that once Obama, the driving force behind the initiative, leaves office in January his successor will keep the issue a high priority. The White House previewed the summit by touting a list of achievements in the U.S.-led effort to tie down loose bomb-grade materials, and arms control advocates commend Obama for galvanizing an international response to the problem. However, many say progress has slowed since the last summit in 2014 and countries such as Japan, India and Pakistan are preparing activities that could increase stockpiles of nuclear materials. “The Nuclear Security Summits have had a positive effect, but the strategic goal of developing an effective global nuclear security system remains unachieved,” the Nuclear Threat Initiative, an anti-proliferation watchdog, said in a report this month. According to the group’s Nuclear Security Index, which tracks the safety of weapons-usable nuclear materials, the past two years have brought no improvement in a range of measures, including on-site physical protection, security during transport and the ability to recover lost radioactive materials. The report also said many countries’ nuclear reactors were vulnerable to online attacks. Seven of 24 countries with weapons-grade material, including China and Belgium, received the lowest possible score for their facilities’ cyber security. Other critics point to a lack of an agreed-upon set of international standards for nuclear security or a mechanism for keeping tabs on common sources of radioactive material often found in hospitals and medical labs. However, Laura Holgate, Obama’s adviser on weapons of mass destruction, cited commitments from 30 countries at the 2014 summit to secure their most dangerous material.  “The international community has made it harder than ever for terrorists to acquire nuclear weapons, and that has made us all more secure,” she told reporters before the summit.   Two of the Brussels suicide bombers secretly filmed the daily routine of the head of Belgium’s nuclear research and development program and considered an attack on a nuclear site in the country, according to Belgian media. U.S. experts are less concerned about militants obtaining nuclear weapon components than about thefts of ingredients for a low-tech dirty bomb that would use conventional explosives to disperse radioactive material and sow panic. U.S. officials said they had no doubt that Islamic State, which controls swaths of Syria and Iraq, was interested in obtaining such materials, but Holgate said U.S. authorities had no “explicit indications” that the group had tried to do so. More commitments from world leaders to enhance nuclear security are expected at the summit but anti-proliferation groups worry that without further meetings at the highest levels, interest could wane and improvements could backslide. 