Robert Wood, a US state department spokesman, said the US embassy in Beijing had lodged a protest with the Chinese government.

"We felt our vessel was inappropriately harassed," he said.

The tense naval standoff comes just two and a half weeks ahead of a summit
between Barak Obama, the US president, and Hu Jintao, the Chinese president, in London.

"Increasingly aggressive"

A Chinese intelligence collection ship challenged the Impeccable over the radio, calling its operations illegal and directing the vessel to leave the area
or "suffer the consequences", the Pentagon said.

Because the vessels' intentions were not known, Impeccable sprayed its fire hoses at one of the vessels in order to protect itself, the Pentagon statement said.

The US ship's commander asked the Chinese ships by radio "in a friendly manner" for a safe path to leave the area.

Two Chinese boats then moved directly in front of the USNS Impeccable, forcing it to take emergency action to avoid collision, said the statement.

The Pentagon said the encounter was the latest incident of "increasingly
aggressive" Chinese conduct in the area.

Marine Corps Major Stewart Upton, the Pentagon spokesman, 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.

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