A leading Soviet commentator today called the U.S. decision to produce neutron warheads "criminal" and said the Reagan administration is following the most openly prowar policy in American history.

Tass political observer Yuri Kornilov rejected the U.S. explanation that the enhanced radiation weapons were intended for use against Soviet tanks in Europe.

"This means the United States is prepared to defend Western Europe with the neutron bomb until the last West German, Belgian, Italian, Dutchman, remains alive," Kornilov wrote.

"This is nothing more than Washington's admission that neutron warheads are meant for use not on the territory of Texas or Arizona, but first of all, on the territory of NATO allies of the United States, in densely populated Western Europe."

Kornilov's criticism was particularly harsh toward the Reagan administration, referring to the "criminal decision" announced last week.

The official Communist Party newspaper Pravda, meanwhile, increased pressure on West Germany today to resist deployment of new nuclear missiles on its territory, warning Bonn the move would place its relations with Moscow in jeopardy, Reuter news agency reported.

The paper said that if the West carried out plans to station the new weapons in Western Europe in 1983, "the first result will be a negative effect on relations between the Federal Republic of Germany and the U.S.S.R."

It also warned for the first time that Moscow could regard West Germany's acceptance of the missiles as breaching a treaty signed between the two states in 1970, ending nearly 25 years of strain.

Under the NATO arms program, more than one-third of the planned 572 U.S. medium-range cruise and Pershing II missiles will be situated in West Germany.

Pravda said the 1970 treaty between Moscow and Bonn pledged both sides to renounce force or the threat of force in their relations. "But are not the new American weapons just such a threat?" the paper asked.

In Rome, two Italian senate commissions announced they will hold hearings to debate U.S. production of the neutron warhead and the growing row about the deployment of cruise missiles in Sicily. Senate President Amintore Fanfani said the foreign and defense commissions of the upper house would interrupt their summer break and meet Aug. 20.

The government has taken no position on the neutron weapons, saying the U.S. decision was an internal matter. But Socialist leader Bettino Craxi, whose party is in the government coalition, said, "Any new bomb is bad news."

In another development, the mayor of Hiroshima, Japan, the world's first atom-bombed city, sent letters to President Reagan and Soviet President Leonid Brezhnev today to protest efforts to develop and produce neutron warheads.

A spokesman for the city said Mayor Takeshi Araki had written describing the production of neutron warheads as an "outrage defying world opinion . . . . The warhead is the most inhumane weapon that has ever been developed . . . ."