WASHINGTON (Reuters) – 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 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.

“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.

Visiting Washington in September, Chinese President Xi Jinping responded to U.S. worries by saying that China had no intention to militarize its outposts in the Spratlys.

Beijing has said their military roles will be defensive, but the head of the U.S. Pacific Command said last month China was “clearly militarizing” the South China Sea with the aim of achieving East Asian hegemony.

The text of Clapper’s letter in response to questions from McCain was published on the news portal of the U.S. Naval Institute. U.S. 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 surface-to-air missiles, coastal defense 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.

He said tank-landing ships had been employed widely in construction work and the landing of civil aircraft at Fiery Cross Reef in January showed the airstrip there was operational and able to accommodate all Chinese military aircraft.

Clapper said China continued its land reclamation in the Spratlys after Aug. 5, when its foreign minister claimed that it had been halted.

While there was no evidence that China has plans for any significant additional land reclamation in the Spratlys, Clapper said there was sufficient reef area in the Spratlys for it to reclaim more than 1,000 additional acres (400 hectares).

The Pentagon has said that Beijing has sought to bolster its claim to nearly all of the South China Sea with island building projects in the Spratlys that have reclaimed more than 2,900 acres (1,170 hectares) of land since 2013.

(Reporting by David Brunnstrom; addtional reportng by Mark Hosenball; Editing by Alan Crosby)