Tension: China's first aircraft carrier, one of the ships involved in the confrontation with the USS Cowper. Tension: Sailors on parade in front of China's first aircraft carrier, which was in the confrontation with the USS Cowpen. Photo: AP

A US Navy guided-missile cruiser had a confrontation with a Chinese military ship in the South China Sea this month, underscoring rising tensions in the region over China's newly declared air defence zone.

The USS Cowpens, operating in international waters, and a Chinese naval vessel "had an encounter that required manoeuvring to avoid a collision" on December 5, the US Pacific Fleet said on Friday. 

"This incident underscores the need to ensure the highest standards of professional seamanship, including communications between vessels, to mitigate the risk of an unintended incident or mishap." 

China was probably angry that the Cowpens may have been trying to spy on China's only aircraft carrier, which was operating in the area, said Dean Cheng, an analyst at the Heritage Foundation's Asian Studies Centre in Washington.

"This was not an accident," he said. "It was deliberate. The Chinese are raising the ante."

The US government lodged protests over the incident with Chinese officials in Beijing and Washington, said a State Department official who asked not to be named. 

The Chinese embassy in Washington did not immediately respond to an emailed request for comment.

Ultimately, officers on the Chinese and US vessels communicated and both warships manoeuvered to ensure safe passage, said a US Defence official who asked not to be identified. 

While the near-collision was resolved peacefully, Dr Cheng said it hinted at the growing risk of confrontation as China sought ways to assert its sovereignty in the region.

China last month unnerved its neighbours by declaring an air defence identification zone in the East China Sea that overlaps with Japan's zone and includes uninhabited islands claimed by both nations.

"I think we're going to see much more tension in the air and on the surface," Dr Cheng said. "In the South China Sea, we've been seeing a steady ratcheting up of pressure."

The Chinese vessel tried to force the Cowpens to stop, causing a military stand-off,  news website Washington Free Beacon said. The Cowpens continued on its course because it was operating in international waters.

After a second Chinese ship sailed in front of the Cowpens and stopped, the US vessel was forced to change course to avoid a collision, the Free Beacon said.

"It's getting dangerous out there," said Patrick Cronin, a senior adviser for the Asia-Pacific Security Program at the Centre for a New American Security in Washington. "Accidents happen. People can get killed out there through these manoeuvres. We need more efforts at ways to tamp down or avert crises as they arise."

Cronin said the incident appeared to be a "tit-for-tat" response to the US refusing to recognise China's new air defence zone.

"We're making them look impotent with respect to the ADIZ," Dr Cronin said, using the acronym for air defence identification zone.

By trying to block a US ship, China was engaging in "coercive diplomacy – it's neither war nor peace", he said.