President Xi Jinping arrives for a working dinner with heads of delegations for the Nuclear Security Summit at the White House in Washington on Thursday. Reuters

Seated with members of his delegation, US President Barack Obama speaks during his meeting with President Xi at the Nuclear Security Summit in Washington. Reuters

UN Secretary-General Ban Ki-moon arrives for a working dinner with heads of delegations for the Nuclear Security Summit at the White House in Washington on Thursday. Reuters

Li Wei, deputy director of the Customs Control and Inspection Department of China Customs, talks on Thursday about the progress at China Customs in enhancing nuclear security during a press briefing at the fourth Nuclear Security Summit in Washington. Chen Weihua / China Daily

Zheng Zeguang, Chinese assistant foreign minister, speaks at a press briefing on Thursday on the bilateral meeting between President Xi and President Obama on the sidelines of the fourth Nuclear Security Summit in Washington. Chen Weihua / China Daily

President Xi (center right) meets with Danish Prime Minister Lars Loekke Rasmussen (center left) in Washington. Xinhua

Xu Dazhe (left), chairman of the China Atomic Energy Authority, chats with US Energy Secretary Ernest Moniz on Thursday while the later visited the China pavilion of the nuclear industry show of the Fourth Nuclear Security Summit in Washington. Chen Weihua / China Daily