"There is every evidence, every day, that there has been an increase of militarization of one kind or another," Kerry stated, adding that the issue poses a serious concern for the United States.
Kerry further noted that the United States had already had "conversations with the Chinese" on the matter, but expressed confidence "that over the next days we will have further, very serious conversation on this."
China and a number of other countries, including Vietnam, Taiwan and Malaysia, have rival claims to hundreds of islands and reefs in the South China Sea.
Beijing has claimed sovereignty over the Spratly Islands and a 12-nautical-mile zone that surrounds them, but Washington has persistently challenged its claims by conducting patrols near the islands citing freedom of navigation.