The Soviet news agency Tass said tonight that U.S. anger at the slaying on board the hijacked Italian cruise ship was "understandable and just."

"The crimes of terrorists, no matter where they are committed, must be punished most severely," a Tass commentator wrote. "And such severity must be shown unfailingly to all perpetrators of such crimes."

The Tass comments, unusually complimentary of an American reaction, were seen as an implicit linking of the killing of a U.S. citizen aboard the Italian liner on Tuesday with the kidnaping and slaying of Soviet diplomat Arkady Katkov in Beirut last week, although the Beirut events were not mentioned.

Western diplomats here said this week that the Soviets have suggested privately that their reaction to the Beirut murder would be "severe." Katkov, consular secretary at the Soviet Embassy, was officially mourned this week at the Soviet Foreign Ministry, where his portrait was on display draped in black.

The Tass account today quoted President Reagan as saying that the killers of Leon Klinghoffer would be sentenced to death if tried in America. It was not clear what American legislation the president was referring to in his remarks in Chicago yesterday.

Tass equated the hijacking of the Italian cruise ship with the case in 1970 of a Soviet father and son who hijacked a commercial Aeroflot flight to Turkey. A stewardess was killed and two crew members were injured seriously.

A Turkish court sentenced Pranas Brazinskas to eight years and his son, Algirdas, to two years in prison. They were released during a general amnesty in 1974.

The Brazinskases, Tass said, are now in the United States, where they were refused political asylum and are living, without entry visas, in New York.

Staff and news service reports gave the following reaction from other countries:

In Bonn, a Foreign Ministry spokesman said at a news conference that West Germany condemned all forms of terrorism and was confident that Italian justice would take its course. He said he could not comment on the U.S. interception of the Egyptian airliner.

Singapore Prime Minister Lee Kuan Yew, completing a visit to the United States, said the American action would be applauded around the world because "it's the right thing to do, it's the moral thing to do, it will discourage hijacking."

In London, British Foreign Secretary Geoffrey Howe also praised the action, saying, "We start by applauding the fact that terrorists are likely to be brought to justice," correspondent Karen DeYoung reported. Howe said he did not see any reason why the interdiction of the plane would be against any international law.

Howe said the hijacking of the Italian cruise ship would have no effect on Britain's plans to hold high-level talks, tentatively scheduled for next month in London, with a joint Jordanian-Palestine Liberation Organization peace delegation that includes Palestinian representatives "who have repudiated terrorism."

One local afternoon daily in Blackpool headlined "Rambo Reagan" with clear approval.

In Johannesburg, South Africa's biggest selling daily newspaper, The Star, said in an editorial, "Hooray for the good guys."