The United States joined Britain yesterday in signing an international agreement to protect the wreck of the Titanic from thievery and damage caused by salvagers and undersea tourists.

The agreement designates the sunken ocean liner as an international memorial and requires signatories to prohibit their nationals and ships from making unregulated dives on the wreck or selling artifacts from it.

"No other shipwreck has stirred emotions like this one," said John F. Turner, assistant secretary of state for oceans and international environmental and scientific affairs. "It is an important step in protecting this scientific and historical treasure."

Turner, speaking in a telephone news conference, said the agreement was an outgrowth of the Titanic Maritime Memorial Act of 1986 introduced by President Ronald Reagan and passed by Congress.

He said yesterday's agreement had been under negotiation by the United States, Britain, Canada and France since 1997 and will not become binding in this country until Congress approves legislation putting it into effect. Britain signed it in November, Turner said. He had no information on French or Canadian intentions.

The Titanic, the crown jewel of Britain's White Star Line, hit an iceberg and sank on its maiden voyage on April 15, 1912, killing 1,523 people. The wreck rests in 12,000 feet of water in the North Atlantic 325 miles southeast of Newfoundland.

The wreck lay unmolested until 1985, when American marine archaeologist Robert Ballard and French colleagues located it with a "remote operating vehicle," or ROV, towed from a research ship. Ballard dived on it with a small submarine the next year.

Since then, the wreck has been visited many times, both by ROVs and submarines. About 5,000 artifacts have been recovered.

Ballard, now an explorer-in-residence for the National Geographic Society, participated in yesterday's news conference from his home in Lyme, Conn., after returning from a trip to reexamine the wreck with an ROV and conduct a photographic survey of it.

"I was surprised at how little had changed" as a result of the elements, he said. On the other hand, he added, submarines "have been landing on the ship and leaving imprints. They can do, and have done, a lot of damage."

He said repeated landings by submarines near Titanic's grand staircase, around the chiefs' and officers' quarters, and on the forecastle had been particularly harmful. In a telephone interview from his home after the news conference, he said his survey had found "a dozen or more" imprints in all.

"They're destroying the deck," Ballard said. "It's just awful, and it's got to stop." Ballard said submarines had taken tourists to visit the wreck to make movies, to sightsee and even to get married. Several Web sites offer visits to the Titanic aboard Russian Mir research submarines for about $35,000. No one answered the phone at the Russian Embassy yesterday.

"The visits have been fairly continuous," Ballard said at the news conference. Besides Russia, he also singled out France as the source of many visits. A spokeswoman from the French Embassy said she knew nothing of the activities of French submarines.

Turner said the new agreement focused on the United States, Britain, France and Canada as the nations most likely to have the "resources to access the Titanic." By ratifying the agreement, he said, nations would "choke off most sources of financing."

Boston University archaeologist Ricardo J. Elia, a specialist in legal questions surrounding marine archaeology, agreed, praising the agreement as "potentially a very important and protective tool."

Elia explained that winning support from France and Canada is important, not only to control the marine technology but also to restrict the ability of submarines to carry nationals from the signatory countries, fly their flags, or allow expedition mother ships to refuel or buy provisions at their ports.

The bow of the Titanic rests on the bottom of the North Atlantic, about 325 miles southeast of Newfoundland. The United States is attempting to stop a string of salvagers and undersea tourists from damaging the site.The giant propeller of the sunken ship lies on the floor of the North Atlantic.