Top Justice Department officials said yesterday the Mafia has been greatly weakened by law enforcement efforts over the past decade, but remains the country's principal organized-crime force.

Robert S. Mueller III, who heads the department's criminal division, said criminal cases have "decimated" the high commands of Mafia families in key cities. But he added, "it's too early to tell" whether equally potent leaders will replace them.

A department report released yesterday said federal law enforcement agencies should continue to concentrate on the Mafia, while working to ensure that other organized-crime groups never attain the same power. Japanese and Chinese crime groups, known for drug trafficking and violence, are increasingly active, according to the report.

The report also mentions rival Los Angeles gangs known as the Crips and the Bloods, motorcycle gangs and a Jamaican group as "emerging organized-crime groups."

The Justice Department's long-term strategy to target mob leaders has relied heavily on the use of the Racketeer Influenced and Corrupt Organizations Act (RICO).

Significant cases include the RICO indictment of Raymond Patriarca, the alleged leader of a Mafia family that operated rackets in Massachusetts, Rhode Island and Connecticut; the 1988 RICO conviction in Philadelphia of Nicodemo Scarfo and most of his confederates, and the labor-racketeering conviction in June of John M. Riggi, alleged head of the powerful DeCavalcante family in New Jersey.

The FBI has said that it used an informer to penetrate the Patriarca organization, bringing about the indictment of 21 men. The U.S. attorney handling the case has said the organization is "the backbone of organized-crime control and influence throughout New England."

Law enforcement officials in New York are trying for the third time to convict John Gotti, the alleged boss of the Gambino organization, the country's largest and most powerful Mafia family. Gotti, 50, twice has been acquitted of criminal charges, but faces a new RICO indictment accusing him and three others of involvement in the murders of three men, obstruction of justice and evading taxes.