WASHINGTON — The United States and China on Friday escalated their dispute over contested territory in the South China Sea, after the Chinese repeatedly ordered an American military surveillance plane to abandon flights over areas where China has been building artificial islands.
The continued American surveillance flights in areas where China is creating new islands in the South China Sea are intended to challenge the Chinese government's claims of expanded territorial sovereignty. Further raising the challenge, Pentagon officials said they were discussing sending warships into waters that the United States asserts are international and open to passage, but that China says are within its zone of control.
The Defense Department planning comes in response to China's accelerated efforts to build new islands in the South China Sea to bolster claims to a vastly expanded area of sovereignty, a direct challenge to the United States and other nations in the region.
The Chinese government expressed "strong dissatisfaction" with the surveillance flights, while urging the United States to cease actions that they said risked increasing tensions. Hong Lei, the Foreign Ministry spokesman, warned that the American flights, which he called "very irresponsible and dangerous," were "likely to cause an accident."
But the Obama administration was adamant in saying that the American Navy surveillance flights were made in international airspace. Officials from the State Department and the Pentagon said that China's land reclamation in the South China Sea was undermining stability in the region and that the Chinese had no business building airstrips on the contested Spratly Islands.
"This land reclamation is going fast," Cmdr. William Marks, the Navy's chief of media, said in an interview on Friday. "Really fast — faster than we ever imagined." He said the Navy had no intention of stopping its almost daily reconnaissance flights.
"We have freedom of flight over international airspace," he said.
The Navy on Thursday released video footage of an incident Wednesday when a Chinese military dispatcher issued eight warnings — in English — to a Navy P8-A Poseidon surveillance plane as it flew over Fiery Cross Reef, the site of an extensive Chinese land reclamation project in the Spratly Islands, a group reefs halfway between Vietnam and the Philippines in the South China Sea. The Navy also released video footage that it said documented the continued expansion of the reefs, which have been turned into artificial islands with airport infrastructure, including a runway.
American military officials took a CNN crew along on the Navy reconnaissance flight on Wednesday; a military official said the decision to take a television crew and to release the video footage was deliberate.
"It's important that the American public, and the Asian public, too, understands what's going on out there," the official said, speaking on condition of anonymity because he was not authorized to speak publicly about a delicate international issue.
A second Defense Department official said discussions were underway on whether the military would increase its naval presence in international waters in the area, including additional patrols using frigates, destroyers and small combat ships. Such a move would most likely further annoy China. Disagreement over the Spratly Islands has continued for several years. The Spratlys are claimed by at least three other countries, including the Philippines, an American ally; and Vietnam, which has sought warmer relations with Washington.
Earlier this year, analysts released satellite images of a concrete runway that China was building on Fiery Cross Reef, one of the Spratly Islands. The runway is expected to be about 10,000 feet long. The construction on Fiery Cross Reef is part of a larger Chinese reclamation project involving scores of dredgers on at least five islands. China is converting tiny reefs, once barely visible above water, into islands large enough to handle military hardware, personnel and recreation centers for workers.
The Chinese government has maintained that the reclamation efforts are meant to serve civilian purposes like providing a base for search-and-rescue operations. But the Chinese military has also said that the reclamation is for "necessary military defense."
A senior administration official expressed concern that China was trying to build up the islands as a way to make a case for Chinese sovereignty — to "create facts on the ground — and we can't allow that." Separately, Daniel R. Russel, the assistant secretary of state for East Asia, told reporters that American military aircraft would continue to exercise the right to operate in international airspace.
Under the leadership of President Xi Jinping, China has asserted claims in the South China Sea that have increasingly butted against Asian allies of the United States. In particular, the issue of territorial boundaries has come into play.
The United States says that foreign aircraft have the right to fly over waters beyond a nation's 12-mile territorial line. China, meanwhile, asserts that foreign aircraft do not have the right to fly within its 200-mile exclusive economic zone without permission.
The contentious issue has flared up before. In late 2013, China set off a trans-Pacific uproar when it declared that an "air defense identification zone" gave it the right to identify and possibly take military action against aircraft near disputed islands in the East China Sea. Japan, which administers the islands, refused to recognize China's claim, and the United States has since defied China by sending military planes into the zone, unannounced.
The islands, known as the Senkaku in Japanese and the Diaoyu in Chinese, are a seven-hour boat ride from Japan, even farther from China. But they, like the Spratlys, have been at the center of ongoing territorial disputes among China and its neighbors.
Pentagon officials say they worry that China will try to declare another air defense identification zone over the Spratlys. "Is this foreshadowing?" one Defense Department official, who spoke anonymously to discuss a national security matter, said on Friday. "Well, we can certainly see them trying."
"The Navy flyover was a measured response, meant to signal to the Chinese that the world is watching — literally — their provocative reclaiming of land that people don't agree that they own," said Andrew L. Oros, an associate professor of political science at Washington College in Chestertown, Md., and a specialist on East Asia. "This idea that China has, that it can just act unilaterally when other people don't agree, has to be confronted."
To build the artificial island, China has dispatched concrete pylons on cargo vessels from the mainland to build a retaining wall and create the island and a harbor, according to Sean O'Connor, a former United States Air Force intelligence analyst who now works as the principal imagery analyst for IHS Janes, a publication that examines military issues.
China has used dozens of dredges that suck up sand and then dump it to make landfill and has constructed concrete manufacturing plants to provide asphalt for runways, he said.
Mr. O'Connor said the Chinese seemed to be particularly focused on the runway. "It looks as though they go over and over it again to make it thick, and can take any kind of plane," he said in a telephone interview. In particular, he said, the Chinese would be interested in landing bombers and fighter jets.