BEIJING >> A senior American naval commander has implicitly accused China of creating “so-called military zones” close to artificial islands it has built in the South China Sea, declaring that such actions are eroding the security of one of the world’s busiest waterways.

In a speech in Honolulu on Monday, the commander, Adm. Scott H. Swift of the U.S. Pacific Fleet, said commercial ships that had previously sailed freely through international shipping lanes were being diverted from areas deemed to be too close to the artificial islands built by the Chinese in the Spratly archipelago.

Swift, who visited China last month, said that routine commercial and military operations in the South China Sea had become subject to warnings, interrupting the freedom of navigation, as well as air rights, to such an extent that the “unilateral assertiveness” was becoming a trend that was “unacceptable.”

Countries in the region are being forced to finance their navies beyond what is needed for self-defense, he said, implying that this was heightening the risk of an arms race.

Although 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 frigate, the USS Lassen, on a freedom-of-navigation patrol within the 12-nautical-mile limit of Mischief 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.

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.

In reaction to Swift’s speech, a spokesman at the Chinese Foreign Ministry, Hong Lei, said Tuesday that “some country” — an obvious reference to the United States — was “deliberately” exaggerating tensions in the region with the purpose of creating “chaos.”

A Pacific Fleet spokesman, asked for specific examples of commercial ships that had been forced to change their route because of orders from the Chinese navy, said he would need time to research the answer.

Tussles between Chinese vessels and fishing trawlers from other nations around the South China Sea have occurred for years. But the Chinese vessels have been emboldened by a new Chinese law this year that requires foreign fishing vessels to obtain permission to enter waters that China claims.

Trawlers from the Philippines and Vietnam have reported being hosed by Chinese water cannons this year and of being robbed at gunpoint. In September, Vietnam said a Chinese vessel had sunk one of its fishing boats near the Paracel Islands, which are claimed by China and Vietnam.

Swift’s remarks came as the BBC reported that a single-engine Cessna 206 aircraft rented by one of its television crews for filming the creation of artificial islands in the South China Sea had been warned by the Chinese navy to turn back as it neared each of three artificial islands.

The plane was identified incorrectly by the Chinese as a military aircraft and ordered to leave the airspace around the islands China has built, the BBC said.

When the plane headed southwest toward Fiery Cross Reef, known as Yongshu Jiao in Chinese, and was close to 20 nautical miles to the reef where the Chinese have built a major runway, a voice came over the radio, the BBC said.

“Foreign military aircraft to northwest of Yongshu Island, this is the Chinese navy. You are threatening the security of our station,” the voice said, according to the BBC account.

When the plane flew toward Gaven Reef and Mischief Reef, also places where the Chinese have undertaken construction, there were similar warnings from the Chinese, the BBC said.

As the BBC plane turned back toward the Philippines, it came across a military aircraft of the Royal Australian Air Force patrolling the region that had apparently received orders from the Chinese to leave.

The BBC reported the Australian pilot saying: “China navy, China navy. We are an Australian aircraft exercising international freedom-of-navigation rights, in international airspace in accordance with the international civil aviation convention and the United Nations Convention on the Law of the Sea. Over.”

In the Australian capital, Canberra, the country’s Defense Department said that an AP-3C Orion had conducted routine maritime patrols from Nov. 25 to Dec. 4 as part of a regular surveillance operation intended to preserve “regional security and stability” in Southeast Asia.

As Washington’s strongest ally in the Pacific region, the Australian government has increasingly expressed concerns about China’s behavior in the South China Sea.

Australia has operated the Orion aircraft from a base in the Malaysian state of Penang for a range of duties in the Asia Pacific region, including during the war in Afghanistan, according to Peter Jennings, executive director of the Australian Strategic Policy Institute in Canberra.

Australia conducts surveillance flights for its own intelligence purposes and not as part of a joint operation with the United States, he said. So far, the flights over the South China Sea have been irregular and conducted without much fanfare, although former Australian Defense Minister David Johnston mentioned them in a speech in Singapore last year, Jennings said.

The tone of Swift’s speech showed that the standoff between China and the United States and its allies over the South China Sea was becoming increasingly difficult to resolve, Jennings said.

“It’s very hard to see how there can be a compromise between China’s assertion of sovereign control over the vast majority of the South China Sea inside the Nine Dash Line and freedom of navigation,” he said, referring to a line drawn by the Chinese that roughly encircles more than 80 percent of the waterway.

Asked on Tuesday about the BBC flight and the warning from the Chinese navy, Hong of the Chinese Foreign Ministry said that China had “indisputable sovereignty” over the Spratly Islands.

© 2015 The New York Times Company