China is undergoing a "massive expansion" of its nuclear arsenal that could see it reach parity with the United States within the next four or five years, a US official has warned.Christopher Yeaw, US Assistant Secretary, also claims China has been carrying out nuclear testing since at least 2020.Yeaw was speaking at the UN Conference on Disarmament in Geneva, the first carried out since the expiration of the New START treaty between the USA and Russia earlier this month.NATIONAL: Design