WASHINGTON — For months, lawmakers and national security hawks have urged President Obama to stand up to China’s land reclamation of disputed islands in the South China Sea. But now that the Obama administration finally has, the White House does not want to talk about it.

In sending a guided missile destroyer late Monday into waters China considers its territory, the Obama administration sought to exercise what officials called the right to freedom of navigation in international waters.

The move was meant to reassure allies in Japan, Vietnam and the Philippines that the United States would stand up to China’s efforts to unilaterally change facts on the ground by building up artificial islands in the Spratly Islands chain.

But even as it was authorizing the naval patrol, which China promptly called a “deliberate provocation,” the White House tried to play down the episode, anxious to avoid escalating a conflict between the nations, a pair of adversarial Pacific behemoths.

The White House directed Department of Defense officials not to say anything publicly about the episode. No formal announcements or news releases alerting the media to the passage of the destroyer, the Lassen, were to go out, White House officials ordered. And if asked, officials were instructed not to speak on the record about the maneuver, administration officials said.

• Since the 18th century, navigators have referred to the Spratlys as “dangerous ground. »
• The Philippines, China, Taiwan and Vietnam have all laid claim to the Spratlys.
• China and Taiwan base their claims on Xia and Han dynasty records and a 1947 map made by the Kuomintang.
• The seabed beneath the Spratlys may hold up to 5.4 billion barrels of oil and 55.1 trillion cubic feet of natural gas.
• Read more of our coverage from The New York Times Magazine.

As a result, Defense Secretary Ashton B. Carter was left flailing during a scheduled appearance before the Senate Armed Services Committee on Tuesday, just hours after the Lassen left territorial waters near Subi Reef, one of several artificial islands that China has built in the disputed Spratly Islands chain.

Senator Dan Sullivan, Republican of Alaska, told Mr. Carter he had initially planned to “express concern” about American “inaction” in combating Beijing in the South China Sea, but changed his mind after hearing that the Navy warship had entered the 12-nautical-mile zone claimed by China.

“Is that true? Did we do that?” Mr. Sullivan asked.

Mr. Carter demurred. “We have said and we are acting on the basis of saying that we will fly, sail and operate wherever international law permits — — ”

Mr. Sullivan interrupted him. “Did we send a destroyer yesterday inside the 12-mile zone?”

Again, Mr. Carter sidestepped the question, and the two men went back and forth a few more times. The exchange prompted Senator John McCain, Republican of Arizona, to express exasperation. “Why would you not confirm or deny that that happened?”

Finally Mr. Carter acknowledged the episode. “I don’t like in general the idea of talking about our military operations,” he said. “But what you read in the newspaper is accurate.”

It was an extraordinary exchange considering the Pentagon had just hours before quietly informed reporters of the naval movement.

“This move seems to have been carefully planned and well executed to mitigate as much risk as possible,” said Derek Chollet, a former assistant secretary of defense for international security affairs. He said the administration might have just wanted to “let our actions do the talking for us.”

In fact, Mr. Carter was simply following White House orders, administration officials said. “Look, we don’t want to make this a bigger deal than it already is,” a senior administration official said Tuesday, speaking on the grounds of anonymity.

China accused the United States of committing a “deliberate provocation” by sending the destroyer into waters it claims as its own.

“China will firmly react to this deliberate provocation,” Lu Kang, a spokesman for the Chinese Foreign Ministry, said at a regularly scheduled news conference on Tuesday.

Chinese authorities summoned the American ambassador, Max Baucus, to the Foreign Ministry on Tuesday evening and told him that the United States should stop “threatening Chinese sovereignty and security interests,” the national broadcaster CCTV said.

The Chinese Defense Ministry said Tuesday night that two Chinese vessels — a missile destroyer, the Lanzhou, and a patrol boat, the Taizhou — had warned the American warship to leave the disputed waters.

Beijing’s response, though heated, essentially repeated language it has used in the past about its sovereignty over the South China Sea.

Interactive Feature: What China Has Been Building in the South China Sea

The Pentagon said that the Lassen stayed within the 12-nautical-mile border of the Spratly Islands chain for less than an hour, and that American surveillance equipment recorded images.

The Spratly archipelago is closer to the Philippines than to China. Satellite images show that China has built Subi Reef into an island, using huge dredging equipment, and that it has started constructing a runway capable of accommodating military aircraft. It has completed another such runway in the Spratlys, on Fiery Cross Reef, and is working on a third.