A U.S. official disbelieved Israel's assurances during the Cold War that it would avoid acquiring nuclear weapons and feared the United States' main ally in the region would spark a Middle East nuclear arms race, documents from that time show.

A 1969 memo reported intelligence findings that ''Israel is rapidly developing a capability to produce and deploy nuclear weapons,'' despite promises it would not introduce nuclear arms to the region.

The memo by Joseph J. Sisco, an assistant secretary of state, was contained in 50,000 pages of previously secret papers from Richard Nixon's presidency, released Wednesday by the National Archives.

The collection draws heavily on national security files during the Vietnam War, arms control negotiations with the Soviets, and the intense superpower competition for influence in the Middle East and beyond.

Documents are thick with minute aspects of the ebb and flow of progress in Vietnam, showing growing worries about the ability of the South Vietnamese government years before it fell, but also seeking encouragement wherever it could be found.

One May 1970 cable marked ''For Confidential Eyes Only'' provided National Security Adviser Henry Kissinger with an inventory of captured weapons, supplies and food. It noted, for example, that the 1,652.5 tons of rice seized so far would ''feed over 6,000 enemy soldiers for a full year at the full ration.''

North Vietnamese troops were fighting on 1 1/2 pounds of rice a day, cut back to 1 pound when necessary, the cable said.

With improbable precision, the memo said U.S. and South Vietnamese forces had deprived their enemy of the ability to conduct exactly 3,779 typical attacks because of the capture of rockets, mortar and rifle ammunition.

Kissinger, in memos to Nixon, expressed concern about the increasing isolation of South Vietnamese President Nguyen Van Thieu, complicating an already unsteady U.S. war effort. He also told Nixon in May 1970, five years before the war ended, that economic chaos, including 30 percent inflation, was a greater risk to the South Vietnamese government than the communists.

To this day, Israel officially neither confirms nor denies its nuclear status and the actual size of its stockpile remains uncertain. But it has long been considered the only nation in the Middle East with atomic weapons.

Researcher William Burr said the memo on Israel's nuclear program sheds light on a little known area of U.S. intelligence.

''For a long time, the U.S. kept secret its assessment of the status of the Israeli nuclear program,'' said Burr, senior analyst at the National Security Archives at George Washington University. The paper shows ''Israel could develop nuclear weapons fairly quickly, something that isn't widely known.''

In the memo, Sisco urged Secretary of State William Rogers to try to curb Israel's ambitions before it was too late.