Independent counsel Lawrence E. Walsh met privately with the House and Senate Iran-contra committees yesterday in an attempt to persuade them either to refuse or delay a grant of immunity to former National Security Council aide Oliver L. North.

But Walsh said afterward that his request is not likely to be granted.

The committees are scheduled to decide today whether to grant Marine Lt. Col. North immunity in an effort to force him to testify. North has asserted his Fifth Amendment right against self-incrimination in refusing to testify before Congress. Committee members have said they believe a grant of immunity is the only way to obtain his testimony.

Rep. William S. Broomfield (R-Mich.) said during yesterday's Iran-contra hearing, "I think the most important thing we can do for our country is to have these hearings move right along. And until we get the principal witnesses, both Oliver North and {former national security adviser John M.} Poindexter, here I think we're not going to know all the answers."

Rear Adm. Poindexter has been granted limited immunity.

Broomfield said later, "I don't want any more delays. We ought to get to the principal witnesses."

Sen. Warren B. Rudman (R-N.H.) added, "I would find it inconceivable . . . that these hearings could ever be complete without the testimony of Col. North."

Walsh has opposed a congressional grant of immunity because testimony provided under that grant cannot be used against North -- either in an investigation or a trial -- unless Walsh had obtained the evidence independently before the grant was issued.

Walsh said it would be "ideal" if North were refused immunity. But, as an alternative, Walsh proposed that the panels "defer" granting immunity "as long as possible, to have the benefit of the testimony of all the witnesses now about to emerge." Walsh said he also suggested that the questioning and testimony of North be "as narrow as possible so the exposure is no worse than it need be."

The chairman of the Senate panel, Daniel K. Inouye (D-Hawaii) told reporters, "The matter of immunity will be resolved within 48 hours."

Committee staff sources have said that even if North receives congressional immunity, he is not expected to testify until July.

North is believed to have played a central role both in the U.S. arms sales to Iran and the efforts to supply the Nicaraguan contras.

On another front, lawyers for North appeared at the U.S. Circuit Court of Appeals here yesterday for another hearing in their effort to have the Ethics in Government Act, under which Walsh is conducting his investigation, declared unconstitutional.

North lawyer Barry Simon argued before a three-judge panel that because the independent counsel is appointed by an appeals court panel to act as a prosecutor and is not accountable to or removable by the president or the attorney general, his activities constitute "a gross violation of the separation of powers."

Guy Miller Struve, Walsh's attorney, argued that the law is constitutional. He added that any possible question about Walsh's appointment should have been resolved March 5 when Walsh was also appointed by Attorney General Edwin Meese III as a Justice Department employe.

House counsel Steven R. Ross told reporters after the hearing he is sure the court will side with Walsh because Congress consulted a number of constitutional scholars in writing the Ethics in Government Act. Constitutional expert Alan Morrison added that were the appeals court to rule against Walsh, he would expect the Supreme Court to hold an "emergency session" to decide whether to overturn the ruling.

Walsh, speaking to reporters after his meetings on Capitol Hill, said he urged the members to obtain information about North's activities from other witnesses rather than by immunizing North.

"I don't know how it will come out," he said. "The committees' concern is with the very important problem of getting to the bottom of these facts as they apply to the most senior officers in government. They are very concerned with the need for that testimony."