China is pushing ahead with the largest-ever expansion of its nuclear arsenal, modernising the atomic deterrent with an eye on any future conflicts with the United States, experts say.
The Stockholm International Peace Research Institute (SIPRI) think tank estimates that China has a stockpile of around 350 nuclear warheads.
But it is growing fast, and could have 1,500 warheads by 2035, according to a Pentagon estimate published in November.
"China appears to no longer be satisfied with just a few hundred nuclear weapons to ensure its security," Matt Korda, of the Federation of American Scientists, told AFP.
Since its first nuclear test in 1964, China has been content to maintain a comparatively modest arsenal and has maintained that it will never be the first to use nuclear weapons in a conflict.
But in recent years, under President Xi Jinping, it has begun a massive military modernisation drive that includes upgrading its nuclear weapons to not only deter foes but also be able to counter-attack if deterrence fails.
"China is undertaking the most significant expansion and modernization of its nuclear forces in the country's history," David Logan, an assistant professor at the US Naval War College, told AFP.
This involves not only ramping up the production of warheads, but also upgrading the ability to deliver them with a nuclear triad: missiles, aircraft and submarines.
"The changes that are taking place or under way are very significant" and "will turn China from a state that has a nuclear retaliatory capability to one that is the world's third major nuclear power", Eric Heginbotham, Principal Research Scientist at MIT's Center for International Studies, told AFP.
"This will mark the first time in history that the big nuclear powers will need to consider not one potential nuclear competitor, but two, and it will have implications for nuclear planning and stability everywhere."
China is "rapidly" building launch facilities for intercontinental ballistic missiles, with more than 300 silos in total, according to the Pentagon last year.
Help from Russia
Experts say there are obstacles to any rapid build-up of China's atomic stockpile -- primarily its limited means to produce the fissile materials needed for warheads.
One possible helping hand could come from Russia.
Beijing and Moscow pledged to step up nuclear cooperation at the recent summit between Xi and Putin.
Top atomic energy officials from Russia agreed to assist China in completing "fast reactors", which can generate fissile material at a much faster rate than they consume it.
Beijing insisted that the agreement was for its civilian nuclear programme, but experts say it could also be used to build up fissile material stockpiles for warheads.
"It would be technically possible for China to substantially grow its plutonium stockpiles with its new developmental civilian fast-breeder reactors using fuel supplied by Russia," Korda said.
"However, there are no publicly-available indications that China intends to do this."
China has "very limited reserves that would constrain a rapid build-up", Gregory Kulacki, China Project Manager at the Union of Concerned Scientists, told AFP.
"According to public information about the pace of the fast breeder programme's development... it will be difficult for China to produce the plutonium they need quickly."
China's assessment of what constitutes a credible nuclear deterrent may also be changing, experts say, and the substantial upgrades to its nuclear forces will embolden it -- particularly over self-ruled Taiwan or in the disputed South China Sea.
Beijing has ramped up pressure on Taiwan, and has recently conducted two major rounds of military exercises around the island -- which it claims as its territory, to be taken one day.