The White House, reacting to a published account of U.S. involvement in a covert operation to kill a prominent Lebanese Shiite Moslem leader, said yesterday that President Reagan had never authorized that or any other assassination attempt.

"The president never did, never has and never will authorize assassinations," said White House spokesman Marlin Fitzwater.

Fitzwater, like other administration spokesmen yesterday, sought to avoid answering questions regarding what Reagan knew about the covert activities of the late CIA director William J. Casey.

He refused to be drawn into a discussion of whether Reagan had signed a National Security Decision Directive or a separate "finding" (a presidential authorization for a covert activity) that set in motion actions leading to the assassination attempt.

Bob Woodward, The Washington Post's assistant managing editor for investigations, writes in a new book about the Central Intelligence Agency that Casey had arranged privately with the Saudi intelligence service to assassinate Sheik Mohammed Hussein Fadlallah, the main spiritual leader of the extremist Shiite group, Hezbollah.

The first 500,000 copies of the book, "Veil: The Secret Wars of the CIA, 1981-1987," went on sale yesterday.

Central Intelligence Agency spokeswoman Sharon Foster said yesterday, "We don't see any value in debating the authenticity of the book, so we're not making any statements."

One of Woodward's other major disclosures in the book was that Casey, just before dying last spring, told him that he knew of the diversion of funds to U.S.-backed Nicaraguan rebels from the secret sale of U.S. arms to Iran. Casey's widow, Sophia, has denied that her husband said such a thing to Woodward.

Yesterday, Rep. Lee H. Hamilton (D-Ind.), chairman of the House Iran-contra investigating panel, said he believed that Casey not only knew of the diversion but was "a primary actor" in arranging for it.

Sen. Daniel K. Inouye (D-Hawaii), chairman of the counterpart Senate committee, said he would not be surprised if Casey had known, but added, "We'll never know because he's no longer with us."

Shortly before the attempt on Fadlallah's life, Reagan had signed a finding authorizing the CIA to train three Lebanese units for preemptive strikes against terrorists.

When the CIA resisted using the units, Woodward wrote, Casey went "off the books" and arranged privately for the Saudis to take effective control of the operation without telling anyone.

On March 8, 1985, a car bomb exploded 50 yards from the apartment building where Fadlallah lived in a Beirut suburb, killing 80 persons and wounding 200. Fadlallah escaped without injury.

The Saudi government has vehemently denied Woodward's account of its involvement in the bid to kill the Shiite leader.

Chairman David L. Boren (D-Okla.) of the Senate Select Committee on Intelligence issued a statement saying he did not think it "appropriate" to comment on specific allegations in the book. Boren said he felt that the important issue now, growing out of the Iran-contra affair and Casey's controversial CIA stewardship, is to establish better congressional oversight of the agency's covert activities.

He said the Senate Rules and Administration Committee last week approved creation of an independent financial unit within the intelligence panel to audit CIA covert activities as part of a general effort to strengthen oversight. This effort will include regular, in-depth review of all covert operations.

The auditing unit, involving three or four persons, was set up to stave off legislation introduced by Sen. John Glenn (D-Ohio) that provides for the General Accounting Office, a congressional investigative body, to conduct outside probes of CIA covert activities, according to Senate intelligence sources.

Boren said the committee also adopted rules to stop unauthorized disclosures to the news media about CIA covert activities, an issue that caused deep distrust between Casey and the intelligence panels throughout most of his tenure as CIA director.

Boren has been eager to repair the damage and to establish a good working relationship with the new CIA director, William H. Webster. He said the committee had made "great strides" in reestablishing trust between the agency and his committee and that this had led to "a very candid sharing of sensitive information."

Also yesterday, Dr. Daniel Ruge, Reagan's former physician, disputed Woodward's report that the president's wounds were far graver and his recovery from a 1981 assassination attempt far slower than the White House disclosed. Ruge, who retired in 1985, said the recovery was "superb" and that he never saw the disoriented president portrayed by Woodward.