ARLINGTON, VA. — Five Chinese vessels, including a Chinese Navy intelligence ship, surrounded and harassed an unarmed U.S. Navy surveillance vessel operating in international waters Sunday in the South China Sea, an act a Defense Department spokesman called "dangerous" and "immature."

In what the Pentagon says is the fourth such incident in less than a week, the USNS Impeccable, a contract ship in the Military Sealift Command carrying a civilian crew, was conducting underwater listening 75 miles south of Hainan Island when five People’s Republic of China vessels surrounded it, waving Chinese flags and telling the Impeccable to leave the area. Several of the Chinese crewmembers visible on deck also disrobed down to their underwear.

"They did what were some rather immature activities," said Pentagon spokesman Bryan Whitman, but was unable to further describe the actions. "I don’t know that they mooned them."

As two of the Chinese vessels closed within 50 feet, the Impeccable sprayed firehoses, but one of the Chinese vessels continued to approach within 25 feet.

Two of the Chinese ships then blocked the Impeccable’s path forcing her into an "emergency all-stop," according to a statement released Monday by the Pentagon.

The vessels were identified as a Chinese Navy intelligence collection ship, a Bureau of Maritime Fisheries patrol vessel, a State Oceanographic Administration patrol vessel, and two small Chinese-flagged trawlers.

On Saturday, a Chinese intelligence collection ship contacted the Impeccable over the bridge radio, the Pentagon said, claimed the Americans were acting illegally and to leave the area or "suffer the consequences."

But Pentagon officials insisted the U.S. ship was operating legally and that the Chinese actions were unprovoked.

"We think these were unprofessional maneuvers by the Chinese vessel, and violations under international law to operate with due regard for the rights and safety of other lawful users of the ocean," said Whitman, adding that formal protest would be lodged later with the Chinese military attache in the Pentagon.

On Sunday, the Chinese crews also attempted to snare a line from the Impeccable that was dragging an "acoustical array" of sonar equipment, and dropped pieces of wood in front of the ship.

In a more serious tone, he later added: "This was not the first time there’s been some provocative maneuvering. ... This is one of the most aggressive incidents that we’ve seen in some time."

According to the Pentagon, on Wednesday, a Chinese fisheries patrol ship shined high-intensity lights on another surveillance ship, the USNS Victorious, and cut off her path in the darkness of night.

On Thursday, a Chinese frigate twice crossed within 100 yards of the path of the Impeccable while Chinese planes buzzed the ship 11 times.

"The bottom line is that this was a reckless and dangerous maneuver that was unprofessional," Whitman said.

The White House said Monday that U.S. Navy ships will continue to operate in the South China Sea.

"We’re going to continue to operate in those international waters, and we expect the Chinese to observe international law around that," White House spokesman Robert Gibbs said during a news conference.

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