A senior Soviet official reportedly has told U.S. Ambassador Malcolm Toon that seven Siberian Pentacostalists will not face prosecution if they leave the U.S. Embassy, where they had lived for 15 months in a bid to emigrate to America.

The official, understood to have been Interior Minister Nikolai Schelekov, is said to have told Toon yesterday that the seven should begin applying for exit visas, leave the embassy and return to their Siberian city of Chernogorsk to complete the application process according to Soviet law.

Until yesterday, the Soviets reportedly maintained there would be no prosecution of the two families only if authorities concluded that no laws had been violated.

Toon was said by an embassy spokesman today to consider Schelekov's words to mark a possible change in the Soviet stance in the long impasse over the fate of the Pentacostalists, who burst past Soviet guards on June 27, 1978, and have refused to leave until Soviet authorities let them go to the United States with their families.

The spokesman said Toon personally reported his conversation to the seven today and strongly advised them to begin the application process and consider leaving the embassy on the basis of what Schelekov said. The interior minister was said to have told Toon the Pentacostalists' visa applications would be considered "in general conformance" to the human freedoms provisions of the 1975 Helsinki agreements and that the embassy would be kept advised of the status of the emigration applications.

Today's meeting was Toon's first face-to-face discussion with the five members of the Pyotr Vaschenko family and Maria and Timofei Chymkalova since they fought their way into the embassy building. Through aides, Toon has repeatedly urged the seven to leave, saying they could not help themselves or their cause by staying.

Pentacostalists, who several times during the past 17 years have gotten into the embassy in hopes of emigrating and then been punished by the Soviets after leaving, have refused to budge. Toon's attitude has been resented by some of the embassy staff as needlessly unsympathetic to the Siberians.

Others have taken the ambassador's side, seeing no way the embassy can aid the families regardless of the issue of the religious repression, since only the Soviets can issue exit visas.

The families said tonight they are deeply skeptical of the Soviet offer, and recounted troubles they already have had from authorities in Chernogorsk, a mining city where religious believers have been harassed and jailed over the years.

"I believe Mr. Toon," said one member of the Vaschenko family, "but I don't believe the Soviet authorities." Vaschenko, a 53-year-old unofficial cleric in his sect, said, "I think going back to Chernogorsk would only drag it out. Who does this help?" He said the family had mailed three exit visa applications to authorities there without response.

There are 13 children in his family; three of them are in the embassy, where the two families live in a one-room basement apartment. They pass the days by reading in English and Russian, listening to the radio, playing board games, talking and praying. Several of the younger members now speak some English.

Mrs. Chymkalova flatly rejected the notion of returning to the mining city, which is located about 2,000 miles east of Moscow. She and her 17-year-old son have already renounced their Soviet citizenship, she said, and she firmly believes the authorities will arrest them if they leave the embassy under these conditions.

One of the Vaschenkos's teen-age sons was seized by Soviet guards at the embassy entrance during the family's struggle to get in. He is in a labor camp. Vaschenko said the family's decision may depend on the fate of this son, but telephone calls to the city to find out his present condition have been unsuccessful in recent days.