America's chief strategic arms control negotiator is likely to return to the Geneva talks this week with new concessions from President Reagan. But Edward L. Rowny, the chief US negotiator in the strategic arms reduction talks (START) with the Soviets, warns that any major new moves toward flexibility ought to come first from them. Ambassador Rowny argues that the Americans have already shown ''a great deal of flexibility'' while the Soviets have shown what he describes as ''some flexibility.''
''The President may or may not arm me with more flexibility,'' said Rowny, who is set to begin a fifth round of negotiations with the Soviets on Friday. ''I think he probably will.''
''On the other hand, it takes two to tango,'' continued Rowny in an interview. ''And we won't show too much more flexibility until the Soviets show some more. . . .''
President Reagan is expected to make a statement concerning the START talks in the White House Rose Garden on Tuesday following a send-off meeting with Ambassador Rowny.
Last week, in a tough-sounding statement, Soviet President Yuri Andropov questioned President Reagan's commitment to arms control and dismissed the President's newest proposals made Sept. 26 at the United Nations on medium-range nuclear missiles.
In testimony prepared for a Congressional subcommittee, meanwhile, Seweryn Bialer, director of Columbia University's Research Institute on International Change, warned that US-Soviet relations are moving in the direction of a ''new cold war.''
But Ambassador Rowny, a retired Army lieutenant general, tended to discount Andropov's blast at the US as part of a propaganda war aimed at preventing the deployment of new US nuclear missiles in Western Europe, scheduled to begin in December.
Rowny said that the biggest problem facing negotiators in the next round of the talks on strategic nuclear missiles was the Soviets' reluctance to give up their advantage in heavy missiles, which provide them, he said, with about three times more ''throw-weight,'' or destructive capability, than the US possesses.
What most worries the Soviets, he said, is the number of long-range, air-launched cruise missiles which the US may be developing. A final agreement might involve a tradeoff of a certain number of US cruise missiles in return for a major Soviet reduction in throw-weight, Rowny said. The US, he added, would also be willing to trade off ''potentially powerful'' submarine-launched missiles, meaning a possible cut in plans to deploy the new, highly accurate D-5 warhead on submarines.
But, said Rowny concerning the throw-weight issue, ''that three-to-one advantage has to be reduced considerably if we are going to have a stable situation and a stable agreement.''
In the meantime, during the past two months of recess in the START talks, the administration has been negotiating more intensively with the US Congress than it has been with the Soviets. A powerful group of US Senators and Congressmen has been advocating a ''build down'' proposal under which both sides would agree that for every new weapon they add to their nuclear arsenals, they would reduce by an even larger number older, more ''destabilizing'' weapons. Administration officials say that while President Reagan has accepted this proposal in principle, assimilating the seemingly simple build-down idea into his START proposals has proven to be a complicated task.
General Rowny said that high-level study by administration officials of the build-down concept was continuing even into ''the last hours'' before he returns to Geneva.
Congressional specialists said, meanwhile, that administration officials were divided over how to incorporate the build-down concept into the President's START proposals. They said Defense Department officials opposed accomodating to the concept in any significant way. But Republican Sen. William S. Cohen of Maine has warned that he and a number of others will not support appropriations for the new MX missile in a key vote expected in the next few weeks, unless President Reagan does adopt the build-down concept.