By JUDITH MILLER
 
n a major expansion of the American-led international effort to stop the spread of unconventional weapons, Panama has agreed to permit the United States to board and search its flagships on the open seas if they are suspected of transporting nuclear, chemical or biological arms or equipment, administration officials said Monday.

The officials said that the addition later this week of Panama, the world's leading shipping registry, and of Liberia in February, to the Proliferation Security Initiative would subject nearly 15 percent of the world's roughly 50,000 large cargo ships to being boarded and inspected on short notice.

The cargo ships carry roughly 30 percent of the world's commercial shipping tonnage, officials said. The initiative is a loose affiliation of countries organized by the United States to intercept unconventional weapons.

One senior administration official called Panama's decision to join the initiative a "significant expansion" of the American-sponsored effort to stop illicit trafficking in nuclear, chemical and biological arms and related equipment and materials through such interdictions.

"With Panama, we now have the world's largest, and with Liberia, the second largest shipper signed on to this effort," said the official, who monitors nonproliferation closely.

Officials said Panama's minister of government and justice, Arnulfo Escalona, and John R. Bolton, the under secretary of state for arms control and international security, would sign the pact on Wednesday.

The inclusion of Panama in the interdiction effort is the latest example of what has been a rapid, but quiet expansion of the initiative President Bush unveiled almost a year ago in a speech in Krakow, Poland. In his speech, Mr. Bush said the United States and 10 allies had begun working on new agreements to seize illegal weapons and to search planes and ships carrying suspicious cargo.

Since then, more than 60 nations have expressed support for the initiative. About nine joint exercises involving the boarding of planes and ships have been held throughout the world, and the scope of the initiative has been broadened. At the fifth meeting of adherents in Lisbon in March, Canada, Norway and Singapore joined the effort as members of the initiative's core group. Participants also agreed to involve their national military and intelligence services and law enforcement agencies in efforts to "shut down proliferation facilitators and bring them to justice."

Officials said representatives of the Treasury and Justice Departments met last week with interagency counterparts to talk about how to expand cooperation.

They also said Russia was interested in joining the core group.

Mr. Bush has made the initiative a linchpin of his multilateral efforts to stop illicit trafficking in unconventional weapons. The seizure in October of the cargo of a German-owned ship, the BBC China, which was carrying centrifuge components to Libya, has been the initiative's biggest single success to date. But officials said that chemicals bound for North Korea were also seized last August by Taiwan and that there had been roughly a dozen such interdictions. Most of them have not been made public to safeguard the identity of those who provided information about suspicious cargo or the way in which such information was collected, officials said.

Under the agreement initialed by Panama on April 15, an amendment to a maritime cooperation agreement aimed mainly at drug interdiction, Panama and the United States can ask each other to board their respective flagships outside their own territorial waters and seize the cargo if it turns out to be related to unconventional weapons programs. Both parties would have two hours to respond to such requests. The government that makes the request must present information supporting its suspicions if asked to do so. If there is no response, the interdiction may proceed, said one official who has monitored the agreement.

Officials said that although the agreement between the United States and Panama was bilateral, other nations would be able to "piggyback" on it.

"With these agreements with Panama and Liberia and the support of more than 60 other nations, the U.S. will have the ability to seek rapid consent to board ships that represent roughly 46 percent of the world's commercial fleet in dead-weight tons," one official said. "This is a big step forward."

Liberia has some 2,000 large cargo ships, that is, ships of over 500 dead-weight tons, registered under its flag, administration officials said. Panama has some 5,600. "That means that the sun never sets on their ships," one administration official said.