The global arsenal of nuclear arms is shrinking. And yet China appears to be the only internationally sanctioned nuclear weapon power that’s increasing its stockpile. China has added about 10 warheads to its nuclear arsenal in the past year, according to a report by the Swedish think tank Stockholm International Peace Research Institute.

The addition of warheads could be troubling considering China’s quickly modernizing military, now the world’s second largest by spending. In a defense paper last month Chinese defense officials omitted a promise it has maintained since the 1960s to never initiate the use of nuclear weapons. And in December of last year, Xi Jinping said China’s nuclear weapons were “a strategic pillar of our great power status. That’s a sharp break from previous Chinese officials who have downplayed the country’s nuclear capabilities, according to James Acton, a senior associate at the Carnegie Endowment for International Peace who wrote an editorial (paywall) on the topic.

China’s arsenal of an estimated 250 warheads is small compared to those of Russia and the US, home to about 8,500 and 7,700 warheads respectively. (Of note: China is considered the least transparent of the official nuclear armed states about its nuclear forces. Some US and Russian academics have said that the country’s arsenal could be much larger, up to 3,000 warheads, but US military officials have dismissed the higher estimates.)

Acton writes that the likelihood of nuclear escalation with China is low and that Beijing may just be responding to security issues like North Korea’s most recent threats of war or increased US military presence in the region. That said, even if the chance of escalation is low, the costs are high, considering China’s territorial rows with Japan, India and several Southeast Asian nations.

At the beginning of the year, the worldwide arsenal was an estimated 17,265 warheads, down from 19,000 at the beginning of 2012. The reduction reflects the weapons reduction by the US and Russia under bilateral arms control treaties. The UK and France left their stockpiles unchanged. Pakistan and India, which aren’t considered nuclear weapon states, expanded their stockpiles and missile delivery. Under the non-proliferation treaty of 1970, only five countries—Russia, the US, France, the UK, and China—are allowed to possess nuclear weapons and considered nuclear weapon states.