The Soviet Union today backed up its new offer to begin East-West talks to limiting medium-range nuclear missiles in Europe with a blunt warning that it is ready for a major arms race if the United States and NATO challenge Moscow's Central European forces.

The Communist Party daily Pravda, in the first detailed official accounts of Leonid Brezhnev's two days of talks last week with West German Chancelor Helmut Schmidt, reasserted that the Kremlin "could agree to" a discussion of medium-range nuclear missiles prior to U.S. Senate approval of the stalled SALT-II treaty.

Such talks, said Pravda, must include both medium-range weapons and U.S. forward based systems -- presumably referring to nuclear-armed U.S. aircraft based in Europe and the Mediterranean. Forward-based systems are to be included in the framework of the SALT-III talks, which themselves could not begin until after SALT-II is ratified.

Pravda said the Soviets told Schmidt that "the U.S.S.R. will not allow the U.S. and NATO to break the strategic equivalence. If the United States deploys additional American missiles in Western Europe, the Soviet Union and its allies will take all measures to restore the balance thus upset."

Brezhnev, said Pravda, flatly rejected Schmidt's view that Soviet deployment of its new, mobile SS20 medium-range missile had upset the balance of forces in Central Europe.

Instead, Pravda said the Soviet president and party chief accused NATO of reaching for nuclear supremacy on the continent by deciding last December to increase its forces with cruise and Pershing missiles.

Until last week, the Soviets had refused to consider talks until NATO suspended that plan. Pravda carefully said the new Brezhnev proposal is "an alternative version," and the earlier demand still stands. In this way, Pravda avoided having to suggest that the new Soviet proposal means the Kremlin was faced down by NATO.

Pravda expressed suspicion of Schmidt's assertions that NATO is genuinely interested in negotiating even as it builds its new arsenal.

"It will take time to see how things work before it can be said to what extent the [German] statement to aid detente [becomes] concrete steps," Pravda said.

The Soviet statement, which adds some details to the generally ambiguous proposal for talks from Brezhnev, comes at a time when Italian Communist leaders are in Moscow with a long shopping list of complaints against the Soviets.The Italians, representing the most important Communist Party in Europe, have been furious with Moscow over the invasion of Afghanistan and internal human rights repressions.

The Pravda amplication also comes as Schmidt and French President Valery Giscard d'Estaing are meeting in Bonn. The two Europeans are the only Western leaders to meet with Brezhnev since the invasion of Afghanistan. The Soviets have sought to portray these two meetings as helpful to detente, even though the West continues condemning Moscow for the invasion.

Moscow "is resolved not to let present tensions develop into a major tragedy," Pravda said, as if reassuring these West Europeans of Kremlin restraint. At the same time, Pravda said, "It was most clearly stated by the Soviet side that we will not leave our friends [the Afghans] alone in trouble."