China will be able to project "substantial offensive military power" from artificial islands it has built in the South China Sea's disputed Spratly Islands within months, the director of U.S. national intelligence said.

In a Feb. 23 letter to Sen. John McCain, chair of the U.S. Senate Armed Services Committee, 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 defense 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.