The Soviet Union has sentenced two Ukrainian dissidents to long prison terms, followed by five years of Siberian exile, in the first penalties leveled against major human rights activists since a wave of arrests began early this year.

According to dissident sources today. Mikola Rudekko. 56, a poet, was sentenced to seven years in a labor camp and five years of "internal" exile which usually means confinement to Siberia. Rudenko founded a Ukrainian dissident group to monitor Soviet compliance with the human rights guarantees of the Helsinki accortigon European security and co-operation, which the Kremlin signed in 1975.

The Carter administration has criticized Soviet performance on human rights and has sought a vigorous review of Moscow's performance under the Helsinki commitments.

Responding to U.S. actions, including direct high-level contacts with Soviet dissidents, the Soviets have charged interference in their internal affairs. President Carter said at his press conference yesterday that "there are difficulties" with the Soviets as a result of his human rights stand.

A high Soviet official called rights activists "a tiny, miserabl bunch of individuals who do not represent anyone who have severed their links with the people," UPI reported from Moscow.

[Renegades from the socialist countries and the whole problem of dissidents' are openly used for delaying further steps toward detente." Boris Ponomarev, an alternate member of the Soviet Politburo, wrote in the July issue of World Marxist Review.]

In the other Ukrainian case, Oleska Tikhy, a teacher and member of the Ukrainian dissident group, was sentenced to 10 years in a labor camp and five years' exile. Both received the maximum penalties permitted by Ukrainian laws under which they were tried. Tikhy had a prior conviction.

The cases of Rudenko and Tikhy are not well known in the West but they were being watched here by both dissidents and Western diplomats for signs of how Moscow intends to deal, with its human rights activists as the formal Belgrade conference to review the Helsinki accord approaches. The Soviet Union and 34 other nations, including the United States, are scheduled to discuss the Soviet and other human rights records in compliance with the accords this fall. Preparatory talks began last month.

The Ukranian group is a branch of a Moscow dissident group that has been virtually eliminated through arrests by the KGB, the secret police, since February. Nine persons were arrested here and in the Ukraine, Lithuania, Armenia and Soviet Georgie in the spring.

Last week, prominent Soviet dissident Yuri Orlov, whose February arrest drew expressions of concern from the White House, was formally charged with anti-Soviet activity, which carries a maximum prison term of three years. He was the leader of the Helsinki monitoring group.

Sources speculated that Rudenko and Tikhy may have been dealt with more harshly as a warning to others and because their cases were not as widely known as others.

The two men, charged with carrying out anti-Soviet agitation and propaganda under article 62 of the Ukrainian criminal code, went to trial in virtual secrecy in the town of Druzhkova, about 400 miles east of Kiev. Dissident sources said the trial, which lasted about eight days, was held in a workers' clubroom and relatives and friends of the two men were barred.

Rudenko published several volumes of verse in the 1950s and 1960s before he was expelled from the official Writers' Union and the Communist Party. He is said to be quite ill.

Gen. Pyotr Grigorenko, a dissident leader, told Western reporters that the seven-year prison term is a "death sentence" for Rudenko.

In addition to Orlov, another dissident who has been arrested and jailed pending trial is Anatoly Scharansky, a computer specialist, who reportedly faces the capital charge of treason.

Another leading dissident, and member of the Helsinki monitoring group. Alexander Ginzburg, has been jailed since February awaiting further legal action.