China acted in an "irresponsible" way in a stand-off with a US naval ship this month in the South China Sea, Defense Secretary Chuck Hagel said on Thursday.

US officials have said that the USS Cowpens, a guided missile cruiser, had to take evasive action to avoid a collision with a Chinese vessel that had come dangerously close in the December 5 incident.

"That action by the Chinese, cutting in front ...100 yards out in front of the Cowpens, was not a responsible action," Hagel told a news conference.

"It was unhelpful, it was irresponsible."

Hagel said the maritime confrontation, the first reported for several years, pointed to the need for clear protocols between the two militaries to avoid a potential clash in the Pacific.

"That's the kind of thing that's very incendiary, that could be a trigger or a spark that could set off some eventual miscalculation," he said.

The two sides needed to work "to have a mechanism to be able to defuse some of these issues as they occur," Hagel said.

"We're working on it," he added.

His comments were the first public reaction by a top official at the Pentagon since the incident two weeks ago.

The near-collision underlined rising tensions after Beijing last month declared an expanded air defense identification zone in the East China Sea, which Washington and its Asian allies have refused to recognize.

General Martin Dempsey, chairman of the US Joint Chiefs of Staff, told the same press conference that American and Chinese military officers have been meeting to draft rules for when the two militaries encounter each other at sea, in the air or in cyberspace.

He said "those working groups have actually been meeting and making some progress."

China has played down the incident, saying its naval forces followed proper procedures

"During the encounter, the Chinese naval vessel strictly followed protocol and handled (the incident) appropriately," said a recent short statement from the Chinese defense ministry.

The statement also said the two defense departments "communicated effectively" during the incident.

Chinese state media, however, had said the US ship posed a "threat."

According to US military officers, the Chinese vessel involved was an amphibious dock ship, part of a flotilla accompanying Beijing's new Liaoning aircraft carrier.

The confrontation occurred in the strategic South China Sea, where Beijing has asserted control over territory claimed by other countries in the region.

The US military has repeatedly vowed to keep operating in international waters and airspace, and has increased its presence in Southeast Asia over the past year as a counter-balance to Beijing's more assertive regional stance.