Officials in the US Nixon administration four decades ago weighed options to pressure Israel to declare the existence of its a nuclear weapons program, documents released by the Nixon Presidential Library show. The memos and tape recordings from the late 1960's and early 1970's showed that US officials concluded the Jewish state was "actively working to improve its capability to produce nuclear weapons on short notice." In an unsigned National Security Council memo, prepared sometime between April 1969 and March 1970, American officials worried that the Israeli nuclear program might make elusive peace with the Arabs even harder to attain. The memorandum, part of a collection of memos and tape recordings released Tuesday by the Nixon Presidential Library, shows efforts to get Israel to sign the 1968 Nuclear Nonproliferation Treaty. This would have required Israel to open itself to international inspection and dismantle any nuclear weapons program it had. Israel resisted, which the memorandum anticipated, "because Israel views its nuclear option on the NPT as an integral part of its national security." Israel would not be easily influenced, the unsigned memorandum predicted. The treaty requires all but five states - the United States, Russia, China, Britain and France - not to develop nuclear weapons. A total of 189 countries are parties to the treaty. The four exceptions are Israel, India, Pakistan and North Korea. Next month, US President Barack Obama will meet with his Russian counterpart, President Dmitry Medvedev, in Moscow and again at a summit of world leaders in Italy. Obama will carry with him a determination to reduce the spread of nuclear weapons. He has said it is "absolutely imperative" that the United States take the lead. Back in the Nixon era, with little sign of progress toward a peace agreement on the horizon, the NSC memo concluded that "Israel's leaders have probably decided Israel cannot afford to surrender the nuclear option." In fact, the document added, Israel preferred to keep the Arabs guessing as to its power to deter attack, while the program provided bargaining power in negotiating a settlement. But the longer Israel would hold out against signing the treaty, it also would reduce prospects for settling the Arab-Israeli dispute, the memorandum said. "We must be prepared whether to make this a crunch issue with Israel and to make it clear that if Israel elects to go the nuclear route it would cause a fundamental change in the US-Israeli relationship." And that, the memo says, includes "our long-standing concern for Israel's security."