Soviet President Leonid Brezhnev has personally pardoned five Jews imprisoned since 1970 for their part in attempting to escape the country by hijacking an airliner.

At the same time, a number of other Jews long barred from emigrating suddenly have received permission to leave. The abrupt actions coincide with a visit here by 17 U.S. congressmen to discuss emigration, trade and strategic arms limitation.

Western observers said the Soviet moves were timed as a gesture to the delegation, which includes Rep. Charles Vanik (D-Ohio), coauthor of the Jackson-Vanik amendment that ties trade credits for the Soviet Union to eased Jewish emigration policies. Vanik was not available for comment last night.

In an unusual action, Brezhnev signed the pardons Monday as president of the Supreme Soviet, or national parliament. Four of the five were freed from prison Thursday, according to one of them. Vulf Zalmanson, 39.

He said he and Boris Penson, 33, already have received exit visas, and that two others Anatoli Altman, 36, and Leib Khnokh, 35, expect to receive theirs Saturday.

Jewish sources in London reported that the fifth man, Hillel Butman, 45, of Leningrad also was freed by Brezhnev.

They were among a group of more than two dozen arrested for planning to take command of a small Aeroflot plane at Leningrad's Smoiny Airport and pilot it to Sweden. They were seized June 17, 1970 and sentenced to prison in four separate trials.

The pardons leave five of the group still in prison: Mark Dimshits, 51, Eduard Kuznetsov, 38, Alexei Murzhenko, Yuri Fyodorov, and Iosif Mendelevich, 31. Dimshits and Kuznetsov originally were condemned to death, but the Soviets spared them after a world outcry. Their sentence run to 1985.

Jewish sources here said that all the others in the plot have now been freed and have emigrated. The five just pardoned were due to serve another 14 months in strict regime prison camps in Siberia.

Among the longtime "refusedniks" who have received sudden persmission to emigrate is Leonid Slepak, 19, son of Jewish activist Vladimir Slepak who is now serving a five-year sentence of exile in the Soviet Far East. Young Slepak, who successfully hid himself from the Red Army draft for more than a year, said today he and his wife and their infant son will leave in early May.

Other Jewish Sources here reported that six Kiev "refusednik" families have received exit permission, along with Felix Aronovich of Leningrad, who has waited seven years.

Zalmanson told the Associated Press here that "nobody expected" the pardsons and that "nobody asked for them."

Rep. Vanik in recent months has said he is contemplating various moves that would have the effect of easing the trade restrictions against the Soviets. He met with a group of refusedniks here yesterday to discuss his proposals and hear their views on them.

The Soviets long have attacked the Jackson-Vanik amendment as an attempt by Congress to meddle in internal Soviet Policies. While asserting that the United States is only hurting its own trade umbalance by withholding most-favored-nation trade credit status from the Soviet Union, the Soviets have made it clear they want the trade status for symbolic reasons of acceptance as much as for the actual trade improvement.

Washington sources reported recently that 4,418 Jews left the Soviet Union last month, a new monthly record which, if continued through the year, could mean almost 50,000 Jews may emigrate in 1979, a record.