WASHINGTON - DOCUMENTS released on Tuesday from Richard M. Nixon's White House years shed new light on just how much the US government struggled with growing public unrest over the protracted war in Vietnam.

The National Archives opened nearly 200 hours of White House tape recordings and 90,000 pages of documents.

A newly declassified memo to Nixon from his secretary of defence at the time reflects just how much the administration felt and discussed public pressure - even as it weighed US geopolitical strategy - in anguished internal debate over war policy.

The seven-page document cautions the president against a proposal from military brass to conduct a high-intensity air and naval campaign against North Vietnam.

Then-Defence Secretary Melvin Laird said such a plan would involve the United States in 'expanded costs and risks with no clear resultant military or political benefits.'

With peace talks 'seemingly stalled in Paris, with combat activity levels reduced in South Vietnam, but with seemingly rising levels of discontent in the United States, we should review the overall situation and determine the best course of action,' the defence secretary writes to the president on Oct 8, 1969.

'The sum total of the considerations ... casts grave doubt on the validity and efficacy' of the proposal from the Joint Chiefs of Staff at the Pentagon, the memo concludes.

At the time, the Nixon administration was secretly conducting a massive bombing of Cambodia to destroy sanctuaries for enemy troops.

In regard to the war generally, 'we must ... act in a fashion which will maintain the support of the American people,' Mr Laird wrote.

The proposed bombing campaign of the Joint Chiefs sought to drive the North Vietnamese back to the negotiating table. The Nixon administration didn't go forward with the Joint Chiefs' plan.

But in December 1972, it launched what became known as the 'Christmas bombing' of Hanoi when peace talks hit a dead end. The effort stirred even more anger with the American public. North Vietnam called it a terrorist act.

Laird became the biggest proponent of the concept called Vietnamisation, urging Mr Nixon to follow through on a policy of troop withdrawals, putting the burden of fighting the conflict on South Vietnamese troops.

The massive B-52 strikes over Hanoi and Haiphong in the last two weeks of December 1972 were a gambit to shock North Vietnam into a serious posture in peace negotiations. The newly released tapes cover the period leading up to the bombing as well as the execution and are expected to include Oval Office discussions about the assault.

The recordings are of Nixon's White House conversations from November 1972 to January 1973 and cover his re-election that fall, steps to bomb North Vietnam and also to make peace with it.

Historians hoped for insights into the 1972 'Christmas bombing,' one of the most controversial acts in a divisive war and the most concentrated air attack of the conflict.

The documents take historians closer than the latest tapes do to the Watergate scandal that gathered force in 1973 and peaked with Nixon's resignation in disgrace in August 1974.

This is the 12th release of Nixon White House tapes since 1980.

More than 2,200 hours of tape recordings from the Nixon White House now are available, according to the National Archives, which joined the Nixon presidential library in releasing the material on Tuesday.

All the recordings in the latest release are being put online while the papers can be seen at the two institutions. -- AP