The U.S. Navy ship that got into a scrape with five Chinese vessels last weekend in the South China Sea was looking for threats such as submarines, presumably Chinese, in waters that China claims as its own, defense officials acknowledged Tuesday.

The United States maintains that the unarmed USNS Impeccable was operating legally in international waters when it was surrounded and harassed by the Chinese craft. Beijing responded hotly to a U.S. protest over Sunday's incident, and neither nation is backing down, even as they prepare for a much-anticipated first meeting between President Barack Obama and Chinese President Hu Jintao next month.

Although they would not be specific about the Impeccable's mission when it was intercepted by the Chinese ships, two defense officials said the ship is designed and equipped for sub-hunting work and was part of a calculated U.S. surveillance operation in the disputed South China Sea.

The officials spoke on condition of anonymity because the ship's capabilities are sensitive. Other U.S. officials have said on the record that the U.S. military will continue to patrol in the South China Sea despite Chinese objections.

A senior U.S. intelligence official said Tuesday the confrontation was the most serious episode between the two nations since 2001, when tensions rose over an in-flight collision between a U.S. and a Chinese plane.

"They seem to be more militarily aggressive," National Intelligence Director Dennis Blair told members of the Senate Armed Services Committee.

"I think the debate is still on in China whether as their military power increases they will be used for good or for pushing people around."

The surveillance ship tows a sonar apparatus that scans and listens for foreign threats that also include mines and torpedoes. The sonar array was deployed at the time of the confrontation, and a U.S. account says Chinese mariners tried to snag it with poles.

The ship is operated by a civilian crew under Navy supervision. It is not a warship or, strictly speaking, a spy ship. Its work is part of a largely unseen cat and mouse game in which the United States tracks foreign submarines on the open seas.

In this case, the sub-hunting took place in a disputed band of water far off the Chinese coastline but within what Beijing considers a 200-mile economic zone under its control. The zone, under international law, gives a state certain rights over the use of natural resources there. That clashes with one of the cardinal principles of America's doctrine of ocean navigation — the right to unrestricted passage in international waters as long as vessels are not encroaching on the economic interests of the country they pass.

"It is our view that we were operating in international waters," State Department spokesman Robert Wood said Tuesday.

While the United States has offered talks on the issue, neither side appears willing to compromise.

"The Chinese do the same thing. It's just that they don't do it around us," said Bonnie Glaser, an expert on the Chinese military and U.S.-Chinese relations at the Center for Strategic and International Studies.

Glaser said the two nations need a better rule book for the disputed area, but she predicted that both countries will try to make sure the diplomatic sniping over the Impeccable does not go too far.

Analysts also noted that the incident, capping a string of provocations in the South China Sea, comes as China nears announcement that it will expand its naval capabilities. China also revealed this week its plans for a nearly 15 percent increase in defense spending this year.

China will have an aircraft carrier "very soon," a top Chinese naval officer told a newspaper last week, fueling speculation over a pending official announcement on the long-awaited project. Meanwhile, a State Department official said Tuesday that the Obama administration was considering whether to raise the matter with Chinese Foreign Minister Yang Jiechi, who was due in Washington on Wednesday to meet with U.S. diplomats.

U.S. defense officials had said the Chinese boats veered so close to the Impeccable that the U.S. civilian crew had to spray one Chinese vessel with a high-pressure stream of water. Stripped to their drenched underwear, the Chinese crew came within 25 feet. When the Impeccable tried to withdraw, U.S. officials said, Chinese boats veered in its path and dropped debris in the water.