US presidential hopeful Hillary Clinton says the proliferation of nuclear weapons is the greatest threat to national security.
Speaking during the first 2016 Democratic party debate she said "the spread of nuclear weapons" and "nuclear material that can fall into the wrong hands" was the biggest security threat facing the country.
Clinton's response echoes the fears of former officials from consecutive administrations - Republican and Democrat - that fissile material could fall into the hands of extremists or hardline regimes.
Her four competitors for the Democratic nomination offered a range of answers on the topic of national security, that reflected renewed unease about international tumult in the Middle East and beyond.
Both former Maryland governor Martin O'Malley and former Rhode Island governor Lincoln Chafee listed problems in the Middle East, including the spread of Islamic extremism.
Bernie Sanders, champion of the left and Clinton's closest rival in the polls, identified climate change as the biggest threat.
"The scientific community is telling us, if we do not address the climate change, transform our energy system away from fossil fuel, the planet we'll be leaving our kids and our grandchildren may well not be habitable."
Ex-senator and Marine Jim Webb also raised problems in US relations with China.
"The greatest strategic threat that we have right now is resolving our relationship with China," he said.
"Our greatest day-to-day threat is cyber warfare against this country."