NEW YORK—China's ambassador to the U.S. warned Washington against "anti-China" alliances and a "Cold War mentality" as Beijing continued to ramp up its objections to U.S. military activity in the South China Sea in advance of a weekend meeting of defense chiefs from the U.S. and Asia.

In an interview here with The Wall Street Journal, Ambassador Cui Tiankai defended China's recent expansion of reefs it claims in the Spratly Islands to include civilian navigation and some military facilities, which has spurred objections from neighbors worried that Beijing intends to militarize the strategically important region, a major maritime thoroughfare.

He sought, as other Chinese officials have in recent days, to tamp down the controversy over the area—known by the Chinese as the Nansha Islands—while putting the blame for it on the U.S. government, which has called on China to stop its building activities and tone down its territorial ambitions there.

"It was very surprising to us that the U.S. has overreacted to the situation and is escalating the situation," Mr. Cui said. "What the U.S. is doing is giving rise to a lot of questions in China….What is the real intention? Is there an attempt to replay the Cold War in Asia?"

In recent weeks, satellite images have revealed the extent of China's building on reefs it controls, in an island chain where Taiwan, the Philippines, Vietnam, Brunei and Malaysia also lay claim to territory that Beijing says historically belongs to China. The U.S. has responded by exploring options for increasing surveillance flights and ship deployments in the region, and U.S. Defense Secretary Ash Carter on Wednesday called China "out of step with…international norms."

The issue is expected to occupy this weekend's security summit of defense chiefs from Asia and the U.S. in Singapore, known as the annual Shangri-La Dialogue.

Mr. Cui noted that criticism from the U.S. has been rising even after a recent visit to Beijing by Secretary of State John Kerry, where the tensions were aired in what the ambassador called a ''very candid, very friendly conversation" with top leaders.

He warned that the recent rhetoric, coupled with the plans for more reconnaissance flights, threatened to dominate U.S.-China relations—"the most important bilateral relationship in the world," he said—despite cooperation on many other fronts, from trade to fighting global terrorism and climate change.

He also acknowledged a peril to regional stability from the territorial tensions.

"If the good prospect of regional economic cooperation is diminished, everybody will be hurt," he said. "Those are the consequences. I don't know if people in Washington, D.C. have ever given serious thought to such consequences."

Mr. Cui said that a theory was developing among some in China—though he didn't subscribe to it—that the U.S. was seeking an excuse to ramp up military activity in the region. U.S. military alliances with other nations aren't seen in China as sufficient reason for U.S. concerns about China's territorial claims, he added.

"Those alliances are anti-China in nature if that is the explanation," he said. "It is most counterproductive and even stupid to have such anti-China policies….You should not do anything that convinces people back in China that you are really directed against us."

He said that although China's purpose on the reefs was primarily civilian, Beijing wouldn't reassure its neighbors by promising not to site weapons in the area.

"Why should we do that?" he said. "Most of the imports and exports of China go through these sea lanes, so stability in the region is of paramount importance to us. But of course we have to defend the facilities on these islands and reefs, so what we are doing is to provide or build up the necessary facilities for self defense, not for attacking others."

Subscribe to WSJ: http://online.wsj.com?mod=djnwires