Maritime experts were given a rare glimpse of the underlying capabilities of the Chinese navy on Sunday, when crewmen involved in a stand-off with a US surveillance ship in the South China Sea revealed the fleet's previously hidden firepower.

The exposure came as the American vessel USNS Impeccable was attempting to defend itself against what the Pentagon claimed was co-ordinated harassment and aggression from five Chinese ships. Being unarmed, the Impeccable turned its fire water hoses against two of the Chinese vessels that had come within 50 feet in a threatening posture.

Then, the Pentagon records in the admirably restrained language of international diplomacy, "the Chinese crew members disrobed to their underwear and continued closing to within 25 feet."

In the annals of great naval battles, the contretemps may not rank alongside Trafalgar or Jutland. But it must be a contender for this year's award for naked aggression.

The Chinese Foreign ministry has remained silent on the incident, which in the circumstances is probably sensible.

According to the Pentagon, the targetting of the Impeccable came at the end of several days in which Chinese naval vessels had been stepping up their harassment in international waters.

The Pentagon insisted the Impeccable was engaged in "routine" and legal operations in international waters.

"The Chinese navy pursues peace and safeguards the security of the country," navy deputy chief of staff Major General Zhang Deshun told China Daily.

Still, the incident has raised alarm as it illustrated how far the Chinese are prepared to go in the name of "defensive" tactics. Right down to their underpants.