Five Chinese ships shadowed and manoeuvred dangerously close to a US naval vessel sailing in international waters in what appeared to be an act of coordinated harassment, the US Defence Department said Monday, according to dpa. The incident took place against the USNS Impeccable on Sunday in the South China Sea, about 120 kilometres south of Hainan Island, and was one of several recent acts of "aggressive conduct" by the Chinese Navy, defence officials said. "Five Chinese vessels shadowed and aggressively manoeuvred in dangerously close proximity to USNS Impeccable, in an apparent coordinated effort to harass the US ocean surveillance ship while it was conducting routine operations in international waters," the Pentagon statement said. The Chinese ships surrounded the Impeccable, with two of them closing to within 16 metres of the US vessel, the Pentagon said. To protect itself, the Impeccable unleashed fire hoses at one of the vessels, and the Chinese sailors responded by stripping to their underwear as the their vessel came closer, the Pentagon said. As the US ship tried to leave the area, two Chinese ships stopped directly in front of the Impeccable, forcing it to an emergency stop to avoid a collision, the Defence Department said. "The unprofessional manoeuvres 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," said Major Stu Upton, a Pentagon spokesman. "We expect Chinese ships to act responsibly and refrain from provocative activities that could lead to miscalculation or a collision at sea, endangering vessels and the lives of US and Chinese mariners," Upton added. US State Department spokesman Robert Wood said the embassy in Beijing lodged a protest with the Chinese government over the weekend. The Impeccable is designed to support the US Navy by using sonar to detect submarines and other undersea threats. The Pentagon said US naval vessels recently have had several encounters with the Chinese, including: - A Chinese intelligence gathering ship threatened the Impeccable through communications on Saturday, calling the operations illegal and directing the Impeccable to leave the area or "suffer the consequences."