Some 123 UN members announced in October that they would launch the UN conference to negotiate a legally binding nuclear ban treaty.
Some 123 UN members announced in October that they would launch the UN conference to negotiate a legally binding nuclear ban treaty, even as most of the world's declared and undeclared nuclear powers voted against the talks.
Britain, France, Israel, Russia and the United States voted no, while China, India and Pakistan abstained.
Even Japan -- the only country to have suffered atomic attacks, in 1945 -- voted against the talks, saying the lack of consensus over the negotiations could undermine progress on effective nuclear disarmament.
The countries leading the effort include Austria, Ireland, Mexico, Brazil, South Africa and Sweden. Hundreds of NGOs back their efforts.
They say the threat of nuclear disaster is growing thanks to mounting tensions fanned by North Korea's nuclear weapons program and an unpredictable new administration in Washington.
Supporters point to successful grassroots movements that led to the prohibition of landmines in 1997 and cluster munitions in 2008.
"I expect that this will take a long time, let's not be naive," Swedish Foreign Minister Margot Wallstrom said at the UN last week.
"But it's very important in these days when you see more of this rhetoric, and also sort of power demonstrations, including threatening to use nuclear weapons."
"Quite a high number of countries are actually interested in saying we have to break the deadlock that has been on this issue for so many years," she added. "So it's also the expression of frustration."
No progress has been made on nuclear disarmament in recent years despite commitments made by the major nuclear powers to work toward disarmament under the 1968 Non-Proliferation Treaty (NPT), said Beatrice Fihn, director of the International Campaign to Abolish Nuclear Weapons, an international coalition of NGOs.
"There was disappointment with the Obama administration, which made some pledges, but then ignored most of them," she said. "And now there are raised worries with the new US president."
Then-president Barack Obama announced a drive in 2009 to reduce the role of nuclear weapons and eventually eliminate them.
But his administration strongly encouraged NATO allies to vote against this year's UN negotiations, saying a ban would obstruct cooperation to respond to nuclear threats from adversaries.
President Donald Trump threatened a nuclear arms race in a tweet shortly before he took office in January, saying "we will outmatch them at every pass and outlast them all."
However, with experience from the campaigns against cluster munitions and landmines, Fihn believes there's a "good chance" a treaty will be adopted, if not necessarily after the first phase of negotiations, which will end in July.
Even with the major nuclear powers boycotting the debate, a treaty would oblige them to revisit their policies sooner or later -- even if, like Russia and the United States, they're currently modernizing their nuclear weapons arsenal.
"Even if major (nuclear weapon) producers don't sign it, they have a big impact," Fihn said of global treaties. "Look at Russia denying using cluster bombs in Syria. Why? They did not sign (the cluster munition ban), but they know it's bad."
No major powers have commented on the start of the talks so far, although the US ambassador to the UN, Nikki Haley, is expected to issue a statement on the sidelines of opening day.
US and French representatives explained their countries' opposition in October citing a need to make progress in stages, without disturbing the current strategic balance of weapons or jeopardizing nuclear deterrence.
Fihn compares such arguments to the logic of chain smokers: "It's never the right time to quit."
"But with the multipolar world, lots of countries feel like they don't have to wait for the superpowers to act," she added.