China will be able to project "substantial offensive military power" from artificial islands in the South China Sea, the director of US national intelligence said.
James Clapper said Chinese land reclamation and construction work in the Spratlys had established infrastructure needed "to project military capabilities in the South China Sea beyond that which is required for point defence of its outposts."
"Based on the pace and scope of construction at these outposts, China will be able to deploy a range of offensive and defensive military capabilities and support increased PLAN and CCG presence beginning in 2016," Clapper said in the letter released this week, using acronyms for the Chinese navy and coastguard.
"Once these facilities are completed by the end of 2016 or early 2017, China will have significant capacity to quickly project substantial offensive military power to the region," Clapper added.
The United States has voiced concerns about China's assertive pursuit of territory in the South China Sea. The sea is one of the world's busiest trade routes and regional countries have rival claims, creating a potential flashpoint.
Asked about Clapper's comments on Friday, Chinese Foreign Ministry spokesman Hong Lei said China has merely been exercising its right of self-defence.
"China has made appropriate and reasonable defence deployment construction that is within the range of China's sovereignty," Hong told a regular news briefing in Beijing.
"China urges the relevant country to not talk excitedly with wild gestures on this issue."
Visiting Washington in September, Chinese President Xi Jinping responded to US worries by saying that China had no intention to militarise its outposts in the Spratlys.
Beijing has said their military roles will be defensive, but the head of the US Pacific Command said last month China was "clearly militarising" the South China Sea with the aim of achieving East Asian hegemony.
The text of Clapper's letter in response to questions from Senator John McCain was published on the news portal of the US Naval Institute. US officials confirmed the content.
Clapper said that while the United States had yet to observe deployment of significant Chinese military capabilities in the Spratlys, it had built facilities able to support them, including modern fighter aircraft.
China had already installed military radars at Cuarteron and Fiery Cross reefs, and the infrastructure could also allow for the deployment of surface-to-air missiles, coastal defence cruise missiles and an increased presence of warships, he said.
The United States had not seen Chinese air force activity in the Spratlys, but warships had stopped at its outposts including a guided-missile frigate and a guided-missile destroyer in December and January, Clapper said.
