Well, yeah - "China’s New Islands Are Clearly Military, U.S. Pacific Chief Says "

Kevin Baron of DefenseOne reports "China’s New Islands Are Clearly Military, U.S. Pacific Chief Says" :

“I believe those facilities are clearly military in nature,” Harris said at the Aspen Security Forum, an annual gathering in Colorado of dozens of top U.S. national security leaders, convened by the Aspen Institute.

In his notably undiplomatic remarks, Harris called on China to show meaningful diplomacy to resolve the territorial disputes. But the four-star admiral also appeared resigned to seeing further construction and eventual deployment of military aircraft and ships.

Fiery Cross Reef sits smack in the middle of South China Sea sea lanes

“They are building ports that are deep enough to host warships and they’re building a 10,000-foot runway at Fiery Cross Reef,” Harris said, referring to one of China’s construction activities in the Spratly Islands that Japan has protested. “A 10,000-foot runaway is large enough to take a B-52, almost large enough for the Space Shuttle, and 3,000 feet longer than you need to take off a 747. So, there’s no small airplane that requires a runway of that length. They’re building rebutted aircraft hangers at some of the facilities there that are clearly designed, in my view, to host tactical fighter aircraft.”

Development on Fiery Cross Reef

I think the Admiral probably said "revetted" and not "rebutted" but that's a minor note. The major note is that -

The top U.S. military officer in the Pacific sternly warned China on Friday to immediately cease its “aggressive coercive island building” in the South China Sea, which he argued was intended clearly for China’s military use as forward operating bases in combat against their regional neighbors.

UPDATE: These maps from an older post might be helpful in visualizing sea lanes a/k/a sea lines of commerce: