Attorney General Benjamin R. Civiletti assured members of the Senate Ethics Committee and the Senate leadership yesterday that the Justice Department is working quickly to resolve separate investigations of Sens. Harrison A. Williams Jr. (D-N.J.) and Howard W. Cannon (D-Nev.).

Civiletti and Philip B. Heymann, head of the department's criminal division, met for about two hours yesterday with Sen. Robert C. Byrd (D-W. Va.) the majority leader, Sen. Howard H. Baker Jr. (R-Tenn.), the minority leader, and members of the committee, which has also been investigating the conduct of the two senators.

Sen. Howell Heflin (D-Ala.), chairman of the ethics committee, declined to comment on the details of the unannounced meeting. But another participant said the senators again expressed concern about the pace of the Justice inquiries.

Heflin noted yesterday that committee members had expressed dissatisfaction earlier with delays in the Justice investigation, and had announced plans to have Civiletti meet with Byrd and Baker.

Williams is among the members of Congress caught in the Abscam undercover FBI investigation. Sources have said he was videotaped several times talking with undercover agents about taking a share in a titanium mine in return for legislative favors.

Williams has denied the charges. Justice officials have told the Senate committee that they expect to reach a decision about whether to seek an indictment of the senator by Sept. 1.

A decision in the case against Cannon, not related to Abscam, is expected a few weeks earlier. Justice prosecutors have been examining his dealings with a Teamsters union associate linked with organized crime at a time when his Senate Commerce Committee was considering a trucking deregulation bill.