The United States expressed concern on Thursday over a report that China is building more than 100 new silos for intercontinental ballistic missiles.
The Washington Post, citing a study of commercial satellite images by a California-based group, reported on Thursday that the silos were being built in a desert near the northwestern city of Yumen.
The James Martin Center for Nonproliferation Studies in Monterey said the 119 construction sites in Gansu province were similar to existing Chinese launch facilities for nuclear-tipped ballistic missiles.
State Department spokesman Ned Price was asked about the report on Thursday.
"I think what is fair to say is that these reports and other developments suggest that the PRC's (People's Republic of China) nuclear arsenal will grow more quickly, and to a higher level, than perhaps previously anticipated," he said.
"This buildup, it is concerning," Price said. "It raises questions about the PRC's intent.
"And for us it reinforces the importance of pursuing practical measures to reduce nuclear risks," he added.
"Despite what appears to be PRC obfuscation, this rapid buildup has become more difficult to hide," Price said. "And it highlights how the PRC appears again to be deviating from decades of nuclear strategy based around minimum deterrence."
Price was also asked whether the United States had a reaction to bellicose remarks by Chinese President Xi Jinping.
Xi, in a speech at centenary celebrations for the Chinese Communist Party (CCP), said "the Chinese people will never allow any foreign forces to bully, oppress, or enslave us."
"Whoever wants to do so will face bloodshed in front of a Great Wall of steel built by more than 1.4 billion Chinese people," he said.
Price said the State Department had "taken note" of Xi's remarks "but we're not going to comment on the specifics."
"I think this administration over the course of several months has been very clear about our impressions of the CCP, in general, but I don't have a specific response for you on President Xi's remarks today," he said.