The Justice Department is investigating allegations that officials of the International Longshoremen's Association received kickbacks from shipping companies to ensure labor peace in major port cities, reliable sources said yesterday.

Two federal grand juries have been convened in New York and Miami to look into charges that ILA officials systematically extorted large sums from shipping firms as the price for preventing disruptions in the loading and unloading of their vessels, the sources said.

Sources said the probe extends to 12 states, with its main focus on ports along the Atlantic Coast and the Gulf of Mexico.

To launch the grand jury inquiries, the sources said, the FBI yesterday began issuing more than 300 subpoenas - mostly for union and shipping company records - in the 12-state area. In addition, the sources added, within the next few days the FBI will start interviewing more than 900 potential witnesses.

Among those identified as having their records subpoenaed are William E. Boyle, an international vice president of the ILA and secretary-treasury of local 1922 in Miami, and Cleveland Turner, head of Miami local 1416.

Thomas Gleason, international president of the 116,000-member union said in a telephone interview from New York yesterday that he had "heard rumors" about the investigation but was unaware of its purpose and scope.

Gleason noted that the Justice Department and other federal agencies have been investigating the ILA for two years in connection with charges that the union conspired to divert cargos to favored shippers.

"I guess this new thing is a spinoff from that investigation," Gleason said. "If you're not getting anywhere I suppose you keep fishing in new areas in hopes that you'll turn up something."