China appears to have completed the first phase of its plan to militarize on the manmade islands its built in the South China Sea.

Construction of runways, hardened aircraft hangars, radar installation and surface-to-air missile (SAM) batteries and other military infrastructure has run its course, said the Asia Maritime Transparency Initiative (AMTI), a think tank based in Washington D.C. AMTI is part of the Center for Strategic and International Studies (CSIS).

This means the units of the People's Liberation Army Air Force (PLAAF) and the People's Liberation Army Navy (PLAN) deployed to these manmade islands now have the capability to launch jet fighters, patrol aircraft and other combat planes at any time.

China also finished building almost two dozen structures on Subi, Mischief and Fiery Cross designed to house long-range SAMs.

AMTI said the military infrastructure on Fiery Cross, Subi and Mischief Reefs in the Spratly Islands include naval, air, radar and HQ-9 SAM batteries on Woody Island. It said satellite images taken this month show new radar antennas on Fiery Cross and Subi.

AMTI said China's three air bases in the Spratlys and another on Woody Island in the Paracel islands to the north allow its military aircraft to operate over most of the South China Sea.

It pointed out that advanced surveillance and early-warning radar stations at Fiery Cross, Subi, Cuarteron Reefs and Woody Island give the PLA broad radar coverage of the South China Sea.

Hardened shelters with retractable roofs housing transporter erector launchers for SAMs were built at Fiery Cross, Subi and Mischief. Hangars on Fiery Cross can accommodate 24 PLAAF combat aircraft and three larger planes, including bombers.

Over the past year, the U.S. Navy has conducted a series of what it calls freedom-of-navigation operations (FONOPS) in the South China Sea, to Beijing's chagrin.

U.S. Secretary of State Rex Tillerson has said China should be denied access to islands it's built in the South China Sea.