Tongsun Park, the South Korean businessman who is a central figure in current congressional influence-buying investigations, has returned unexpectedly to Seoul. Sources here said he went back to visit his seriously ill mother.

There were other indications, though, that Park may also have returned to Korea because his business ventures there are in difficulty.

Park's departure from London, where he has been living since leaving Washington last fall when the Justice Department intensified its investigation of his affairs, raised the possibility of renewed efforts by U.S. authorities to have him returned here for questioning. Both Justice and congressional investigators are anxious to ask Park about the lavish parties, cash and gifts he bestowed on members of Congress.

Attorney General Griffin B. Bell said last month that the White House was aiding the Justice Department in its effort to obtain Park's testimony. But officials at the White House, the State Department and the Justide Department on declined comment yesterday on what new steps, if any, they will take to persuade the South Korean government to help get Park back to the United States.

South Korean government officials consistently have denied that Park had any connection with the regime of President Park Chung Hee. Thus they said they had no control over his whereabouts.

Park's trip to Seoul thus could be viewed as a good sign for U.S. authorities because the South Korean government would be able to put direct pressure on him to cooperate.

On the other hand, the United States had no formal extradition treaty with South Korea, so if Park were detained or chose to remain there, the Justice Department would have no way to force his return.

Wire service reports from Seoul first reported that Tongsun Park had arrived back in his native land. Associates in Washington confirmed later yesterday that he left London hurriedly after receiving a call that his mother had had a stroke and was in a coma.

One person in frequent contact with Tongsun Park said he was concerned the flamboyant South Korean might be in some personal danger in Seoul and wondered about the validity of the report of his mother's illness.

Another long-time business associate said that Tongsun Park has attempted to return to South Korea some months ago because of business problems but had been persuaded not to go. Among other things, Park told friends then he was in danger of losing a lucrative contract to provide crews for ocean-going ships.

Though Park has continued to live in high style in London, business acquaintances say he has been in financial difficulties, running up sizable debts all across the city.

He has been operating there out of an office at 44 Green Street, under the name Eastern Navigation. The company is known to have been working on arranging an irrigation project in Morocco and a land reclamation deal in Abu Dhabi.

William G. Hundley, Park's attorney, said yesterday that "in view of the circumstances, the illness, I would certainly expect that he would just go visit his mother for a few days then depart unhampered."

Some South Korean officials have denounced Park privately for the bad publicity his activities have brought the country. And some of his activities, such as helping other South Koreans evade currency restrictions, could make him liable to arrest there.

Associated said Park planned to return to London by the end of the month. U.S. authorities have made no effort to extradite him from England, saying they don't have enough evidence to indict him for a felony covered by the extradition treaty.

Benjamin R. Civiletti, head of the Justice Department's Criminal Division, said yesterday the department has hoped to avoid what could be a months-long extradition process.

"You can be assured that we are continuing our efforts to get Mrs. Park to return to the U.S. and tell us truthfully about his activities," Civiletti said.

Meanwhile, the House committee investigating the Korean connection moved yesterday toward a legal confrontation with a reluctant witness, Suzi Park Thomson, who allegedly worked with Tongsun Park in the influence-buying effort.

The Committee on Standards of Official Conduct notified Thomson that she may be cited for contempt of Congress if she persists in her refusal to answer certain questions.

A vote on the contempt citation could come as early as next Wednesday, when the committee has scheduled its first meeting since the Congressional recess began. But Thomson's lawyer, Phillip Hirshkop, said yesterday "we'll probably reach an accomodiation" before then.

Thomson worked as a staff aide to five House members, including five years in the office of former Speaker Carl Albert (D-Okla.). She frequently entertained congressmen at large parties, using those occasions to introduce the members to Korean government officials.

Thompson has been questioned twice in closed committee sessions. House sources said she had refused to answer questions about gifts and payoffs offered to members by Tongsun Park and by Kim Dong Jo, a former South Korean ambassador to the United States.

Thompson has said she will answer such questions only in public sessions. To answer in private, she says, would subject her to selective leaks that might reflect badly on her character.