On the eve of a crucial session of U.S.-Soviet talks on limiting medium-range missiles in Europe, U.S. arms negotiator Paul H. Nitze said today that President Reagan has decided the talks should go forward despite the "irresponsible Soviet action" in reportedly downing a Korean Air Lines jet last week.

At the same time, in a statement on his arrival at Geneva for resumption of the negotiations, Nitze called Soviet President Yuri Andropov's Aug. 26 offer to destroy part of the Soviet intermediate-range missile force in return for U.S. renunciation of the planned deployment of 572 cruise and Pershing II missiles in Western Europe "only a limited step forward."

Soviet delegate Yuli Kvitsinsky, who arrived here yesterday, said in a statement that the airline incident "has no bearing" on the talks, which resume Tuesday after a two-month recess.

His statement concentrated instead on Andropov's new proposals as paving the way to agreement in the talks, which NATO has made a condition of halting the deployment in Europe this winter of the U.S.-made missiles.

Nitze, who said that there is "unavoidable linkage" between the alleged Soviet downing of the South Korean jetliner in which 269 persons died and the new talks session, added that "President Reagan has decided that, nevertheless, we should go forward with constructive efforts to achieve arms reductions."

Andropov proposed to "liquidate" Soviet medium-range missiles in the European part of the Soviet Union--including a "considerable number" of SS20s--in excess of the 162 comparable missiles deployed by Britain and France. His proposal was conditional on the United States renouncing the deployment agreed by NATO in 1979 for later this year in the absence of an agreement at Geneva. The Soviets reportedly have 250 of the highly mobile SS20 missiles based in the western part of the Soviet Union.

The Andropov proposal "doesn't really address the essential problem because it would continue the freedom to them to continue to produce SS20," Nitze said.

The U.S. envoy said he had stopped off on his way to Geneva today to consult with West German Chancellor Helmut Kohl, who confirmed his "strong support" for the U.S. negotiating position. "Solidarity within the NATO alliance remains as solid as ever," Nitze added.