Israel has asked President Carter to press Soviet President Leonid Brezhnev at this weekend's summit for information on the fate of Raoul Wallenburg, the Swedish diplomat who saved thousands of Jews in Hungary during the World War II and who is said to be in a Soviet prison.

Prime Minister Menachem Begin, joining an effort by international Jewry to free Wallenburg - if he is still alive - said he instructed Israel's ambassador to the United States, Emhraim Evron to ask Carter to raise the Wallenburg issue in Vienna.

After first denying that Wallenburg was imprisoned , the Soviet Union in 1957 informed the Swedish government that Wallenburg died of heart attack in the Lubyanka prison. Relatives of Wallenburg said today, however, that Soviet prisoners who have been released reported seeing Wallenburg in prison as recently as 1975.

Jan Kaplan, a Soviet Jew who last year was released briefly from a Soviet prison, telelphoned his sister in Tel Aviv and told her he met in 1975 a Swedish prisoner who had been detained since 1945, when Wallenburg was taken into custody by Russians in Budapest. No other Swedish national is known to have been imprisoned that long.

Nina Lagergren and Guy von Darel of Sweden, Wallenburg's half sister and half brother, said in a news conference here that "the Israeli people, the Jews of the world, are entitled to know what happened to the man who went on a mission of humanity."

Wallenberg, who is not a Jew, has been honored in Israel with the title of "righteous gentile" given to persons who assisted Jews during the Nazi pogroms.

Wallenburg's family has formed committees in the United States and Britain to pressure the Soviet Union to acknowledge the former diplomat's presence, and to free him.

Wallenburg, who would be 66 now, was an architect who went to Budapest with diplomatic credentials in 1944 after the Nazis began deporting Hungary's 700,000 Jews to extermination camps.

He printed thousands of Swedish passports and distributed them to Jews for emigration to Sweden, while building an elaborate relief organization with hospitals and shelters over which flew the Swedish flag.

Apparently suspecting him of espionage, the Russians arrested Wallenburg in January 1945. Through the remaining years under the Stalin, the Soviets denied they held Wallenburg, but four years after Stalin's death, Soviet Foreign Minister Andrei Gromyko announced that a death certificate had been found in prison archieves purporting that Wallenburg died in 1947. CAPTION: Picture, RAOUL WALLENBERG . . . Swedish diplomat aided Jews