The United States and the Soviet Union should review their nuclear fail-safe systems, the chairman of the Senate Armed Services Committee said Thursday. ``If nationalists or terrorists overran a Soviet nuclear storage site in one of the republics, would the superpowers coordinate their actions?'' Sen. Sam Nunn, D-Ga., asked in a speech on the changing threat worldwide. Ethnic unrest in Azerbaijan and Lithuania's declaration of independence March 11 have led Moscow to resort to military force. The Bush administration, Nunn said, should urge the Soviet Union to join the United States in conducting separate, but parallel reviews, of fail-safe and control programs. Nunn also suggested using the Nuclear Risk Reductions Centers for talks about how the two superpowers could avoid an inadvertent nuclear war. In his second major speech in a week, Nunn acknowledged the lessening Soviet conventional threat and questioned whether the Bush administration is correctly characterizing the Soviet's modernization of its strategic forces. Defense Secretary Dick Cheney has said unequivocally that there has been no change in Soviet strategic forces and that the only change at all has been in improving their forces. ``The administration has tended to overstate its case,'' Nunn said. U.S. intelligence, he said, has found a cutback in the Soviet's Blackjack bomber production. In addition, the Soviets have ceased patrolling U.S. coasts with its Yankee-class SSBM and patrolling north of Canada with its nuclear-armed Bear-H bomber. In his speech, delivered on the Senate floor, Nunn also discussed: _His complaints last week that the administration's fiscal 1991 defense budget has several major holes Congress will fill if the Pentagon doesn't. Cheney disputed some of Nunn's allegations and said the Pentagon is accounting for rapid changes in Eastern Europe in fashioning a spending plan for fiscal 1992 to 1997. ``It is crucial to note, however, that Secretary Cheney made no claim that the administration's fiscal 1991 budget request reflects all these assumptions,'' Nunn said Thursday. Questioned about Nunn's comments, Pentagon spokesman Pete Williams said Thursday: ``It would be very difficult, if not _ dare one say _ imprudent, to send up a '91 budget on the basis of things that haven't happened yet. As events change, we will adjust our plans accordingly.'' _The likelihood of the Soviet launching a ``go-it-alone'' attack on Western Europe. The chances of such an offensive is remote, according to Nunn, who also said the Kremlin would have a difficult time re-establishing ``a credible threat of a large-scale conventional attack on NATO.'' _The dispute within the Bush administration over the potential for reversing the dramatic events in the Soviet Union and the Warsaw Pact nations. CIA Director William Webster has told Congress it will be difficult; Cheney disagrees. Shortly after Webster's comments, Cheney said the CIA director's testimony ``was not helpful'' to Bush's policies.