by David Morgan

WASHINGTON - The United States on Monday urged China to observe international maritime rules after the Pentagon said five Chinese ships, including a naval vessel, harassed a US Navy ship in international waters.

The Chinese vessels "shadowed and aggressively manoeuvred in dangerously close proximity" to the USNS Impeccable on Sunday, with one vessel coming within 7.6m of the US ship, a US Defense Department statement said.

Defence officials said the incident followed days of increasingly aggressive Chinese conduct in the area, including fly-bys of two US Navy ships by Chinese maritime surveillance planes.

'Our ships operate fairly regularly in international waters where these incidents took place,' White House spokesman Robert Gibbs told a news conference. 'We are going to continue to operate in those international waters and we expect the Chinese to observe international laws around them.'

The US embassy in Beijing lodged a protest on Sunday with the Chinese government, State Department spokesman Robert Wood said.

US defence policy officials on Monday reiterated the protest to China's defence attache in Washington.

Officials at the Chinese embassy in Washington did not return calls seeking comment.

'The unprofessional maneuvers by Chinese vessels violated the requirement under international law to operate with due regard for the rights and safety of other lawful users of the ocean,' Marine Corps Major Stewart Upton, a Pentagon spokesman, said in a statement.

'We expect Chinese ships to act responsibly and refrain from provocative activities that could lead to miscalculation or a collision at sea.'

The Pentagon identified the Chinese vessels in Sunday's incident as a navy intelligence ship, a bureau of maritime fisheries patrol vessel, a state oceanographic administration patrol vessel and two small Chinese-flagged trawlers.

The Impeccable is one of five ocean surveillance ships that serve with the US 7th Fleet, which is based Yokosuka, Japan.

The ships use low-frequency sound to search for undersea threats including submarines, a US military official said.

Hainan Island is the site of a Chinese naval base that houses ballistic missile submarines, according to independent analysts.

The Pentagon said the Impeccable had a civilian crew when it encountered the Chinese vessels on Sunday. The ship's official Web page, maintained by US Military Sealift Command, lists a 50-member crew, including 25 military personnel.

The Chinese vessels surrounded the Impeccable while two closed to within 15.2m, waving Chinese flags and telling the US ship to leave the area, the Defense Department said.

The Chinese used polls in an attempt to snag the acoustic array that allows Impeccable to monitor underwater events, a defence spokesman said.

The Impeccable responded by spraying one of the vessels with fire hoses and its crew disrobed to their underwear.

The US ship informed the Chinese vessels by radio that it was leaving the area and requested a safe path to navigate. Two of the Chinese vessels stopped directly in front of the US ship and dropped pieces of wood in its path.

The Pentagon described accounts of half a dozen other incidents dating back to March 4, in which the Impeccable and its sister vessel, USNS Victorious, were subjected to aggressive behaviour, including dozens of fly-bys by Chinese Y-12 maritime surveillance aircraft.