Although Admiral Swift did not say as much, it was clear he was referring to China.

By mentioning “so-called military zones,” he was offering a counterpoint to an assertion by President Xi Jinping of China during a visit to Washington in September that China did “not intend to pursue militarization” of islands in the South China Sea.

The admiral’s speech, delivered to a regional security forum, was his second concerning the South China Sea since October, and was tougher in tone than his previous warnings about the situation. Officials from China were invited to the forum but did not attend, a Pacific Fleet official said.

The South China Sea has become one of the most serious strategic problems between China and the United States, largely because Washington challenges Beijing’s right to enlarge tiny specks in the waterway into islands big enough to accommodate military runways and radar equipment. The waterway is important to China militarily and commercially: About 75 percent of its oil imports arrive by sea, according to China Energy News, a state-run publication.

In October, the United States dispatched a guided missile destroyer, the Lassen, on a freedom-of-navigation patrol within the 12-nautical-mile limit of Subi Reef, one of the new artificial islands in the Spratly archipelago claimed by China. Other claimants of reefs and islands in the Spratlys include Malaysia, the Philippines, Taiwan and Vietnam.

Admiral Swift said that fishermen from the region were also threatened.

“Intimidated by the manner in which some navies, coast guards and maritime military enforce claims in contested waters, fishermen who trawled the seas freely for generations are facing threats to their livelihoods imposed by nations with unresolved, and often unrecognized, claims,” he said.