AMERICAN officials are losing patience with China. On May 22nd the vice-president, Joe Biden, was blunt. He warned naval-college graduates of “new fault lines” emerging between the great powers. China, he said, was challenging freedom of navigation in the South China Sea by reclaiming land on disputed reefs on a “massive scale”. Two days earlier America had signalled its annoyance by sending a surveillance aircraft close to one of the reefs where China is building an airstrip. Such secretive flights are common, but this one was different. The plane also took a crew from CNN, which broadcast the Chinese navy’s testy response through a radio transmission in English: “Leave immediately, in order to avoid misjudgment.”

Chinese officials and state-controlled media have reacted angrily to America’s rhetorical offensive (reinforced by CNN’s dramatic footage of the spyplane mission over Fiery Cross Reef, showing sand being sucked from the bottom of the sea and sprayed onto the island-in-the-making by Chinese dredgers). On May 25th a Chinese foreign ministry spokeswoman called on America to end its “provocative behaviour”. Global Times, a state-owned newspaper known for its hardline views, said war would be “inevitable” if America kept complaining about the island-building. On May 24th the People’s Daily, the Communist Party’s main mouthpiece, warned America that those who “hurt others” could “end up hurting themselves”.

Mercifully, the heated words have not so far been matched by hot-headed military behaviour in or above the sea itself. Both America and China are anxious to avoid clashes. But in order to make its point, America is considering moves that might be construed by China as threatening. American spy flights, as well as similar missions by its ships, have so far kept at least 12 nautical miles (22km) away from the reefs they are monitoring. That would be the outer limit of China’s sovereign domain if the reefs were islands (ie, permanently above sea) and were indeed Chinese. Now the Pentagon is considering whether to probe these lines.

China has long said it owns most of the reefs and islands in the South China Sea, and has also asserted vaguely defined rights to most of the sea itself. Other countries around the sea dispute these claims (Vietnam and the Philippines both say they own Fiery Cross Reef). America takes no position in the sovereignty debate, but it says arguments should be resolved peacefully, without affecting freedom of navigation. China’s warnings to American spyplanes suggest that it is already trying to impose restrictions on military traffic.

Despite America’s increasingly public complaints, there has been no change in the frantic pace of China’s reclamation efforts on several reefs (pictured is an American spyplane’s photograph of work on Fiery Cross Reef). On May 26th China’s defence ministry released a “white paper” on military strategy. It said the country should build a “modern maritime military force” to protect China’s “maritime rights and interests”, including in the South China Sea. Ash Carter, America’s defence secretary, said a day later that China’s actions in the area showed it was out of step with “international norms that underscore the Asia-Pacific’s security architecture”.

America and China’s neighbours worry that China may eventually declare an “Air Defence Identification Zone” (ADIZ) over the South China Sea—requiring aircraft to identify themselves to the Chinese authorities before entering. In November 2013 China alarmed the region by establishing an ADIZ over the East China Sea, covering islands claimed by Japan (see map). It said its armed forces had the right to take “defensive emergency measures” against those failing to comply. America quickly sent two unarmed B-52 bombers through the zone without notifying China. Some Chinese experts believe that China is unlikely to declare an ADIZ in the South China Sea soon because it would be even harder to enforce over such a vast area. On May 26th, however, a Chinese foreign ministry official said his country would decide whether to establish one partly on the basis of “whether and to what extent the security of airspace is threatened”—a clear warning to America.