FBI Director William H. Webster said yesterday that a small number of Cuban intelligence agents, criminals and prostitutes have been discovered among the flood of Cuban refugees entering Key West.

Webster told a luncheon group of Washington Post editors and reporters that a team of FBI agents has been screening the Cubans who fled Fidel Casto's rule in the past week. He declined to say how many Cuban intelligence operatives have been indentified. "So far it's been pretty small, but we confirmed some," he said.

He also noted that "Castro appears to be preempting a certain percentage of the available seats on those boats for people he doesn't want in Cuba anymore."

Verne Jervis, a spokesman for the Immigration and Naturalization Service, said that about a dozen such "undesirables" have been discovered. Some were detained at the federal correctional center in Miami, he said.

Webster said the screening has covered as many as 1,100 Cubans in one day. "You can figure out how intensive the inquiry can be on that basis," he said.

The FBI director said his agents were working with the Central Intelligence Agency and other inteligence agencies in the screening process. "We have certain profiles we're looking for," he said.

Other FBI officials said it wasn't surprising to find criminals or intelligence agents among the mass of Cubans arriving in Florida because the same thing happened during the great influx of Cubans into the United States in the 1960s.

Webster said the discovery of a handful of undersirables isn't a major concern yet. "The big problem is the lack of control over people coming into the country. It's not our [the FBI's] problem. T's going to be city problem and an INS problem."

He also noted that there has been no sign so far the Cuban terrorists are arriving in Flordia.

The FBI has recently increased its attention to an anti-Castro terrorist group calling itself Omega 7, which has claimed responsibility for several bombings in recent months.

"These [people arriving] are largely refugees," Webster said. "The sympathies of most lie with the Cuban nationalist movement, I suspect. But I don't think we've got anybody there who has linked up with them [Omega 7]."