A petroleum engineer who fled from the Chinese Consulate in New York in April to seek asylum in the United States has mysteriously returned to China.

A State Department spokesman today confirmed that Zhang Zhenggao, 47, had returned to his country. Friends of Zhang said he apparently was kidnaped by officials of the Chinese Consulate in New York.

A Chinese government spokesman said today that Zhang returned home voluntarily after receiving a letter from his wife. But Zhang's friends and his American attorney said much of the evidence indicates otherwise.

Zhang vanished in July from his apartment in Brooklyn. His disappearance is believed to be the first incident of its kind involving any of the more than 1,000 visitors and scholars from the People's Republic of China who have sought political asylum in the United States.

"I am sure he was kidnaped," said Yang Huaian, a friend and fellow defector who lived near Zhang in New York.

Yang said he was the first to discover Zhang missing on July 20. He found the door of Zhang's basement apartment locked from the inside. Left behind in the apartment were clothes, a new suitcase, shoes, a television set and a video cassette recorder.

Yang said Zhang, who escaped from the Chinese Consulate in New York April 12 by lowering himself down a fire hose, had never expressed any interest in returning to China despite receiving letters from his wife and two children in Chengdu, the capital of Sichuan Province.

Instead, Yang said, Zhang had asked for help in reaching Taiwan authorities after recent immigration court decisions against other Chinese defectors led him to fear that his application for asylum in the United States might be rejected.

Dr. Wang Bingzhang, a human rights activist who arrived in the United States in 1982 and who heads the New York-based Chinese Alliance for Democracy, said he had received reliable information from sources inside China that Zhang was convicted of treason shortly after returning to Peking July 20 on Civil Aviation Administration of China (CAAC) flight 982 from New York.

Wang said he also had an "unconfirmed" report that Zhang had been executed. A spokesman for the Chinese Consulate in New York, identifying himself as "Mr. Hou," said today he had no information on what had happened to Zhang after his return to China.

Robert Belluscio, a Flushing, N.Y., attorney representing Zhang on his asylum petition, said an FBI agent told him that Zhang's return appeared to be voluntary, but offered no evidence other than statements from the Chinese Consulate.

Belluscio said Zhang never asked him to withdraw the asylum application.

Zhang had talked with FBI agents several times, Belluscio said, even after he advised his client not to.

"Did the U.S. government help to get him back in touch with the Chinese? Did the Chinese trick him? Those are all questions that I think should be answered," Belluscio said.

A spokesman for the FBI's New York field office said today that he had no information.

Zhang arrived in the United States April 5 as part of a delegation studying U.S. oil-drilling methods. He left his group without permission for two days and underwent lengthy interrogation when he returned to the Chinese Consulate in New York, Belluscio said.

At about 2:30 a.m. on April 12, Zhang lowered himself from an eighth-floor window and fell about three stories to the roof af an adjoining building, where his cries alerted police who took him to a hospital.

His well-publicized escape led to offers of financial support from members of the Chinese community in New York.

Belluscio and Yang said Zhang complained of his inability to get a work permit and the slow progress of his asylum petition. Yang said Zhang disliked his basement apartment, lent him by a benefactor, "because it was too exposed and everybody could see his activities."

Wang said his sources in China, which include children of high Communist Party officials, told him that four Chinese Consulate officials visited Zhang July 15 and told him that they wanted to take him to meet a friend in New York. Belluscio said Zhang apparently left the apartment by a rear bathroom window, which he could not have done without help.

Instead of meeting the friend, Wang said, Zhang was taken to the consulate and kept there until he was escorted to the Chinese flight from John F. Kennedy International Airport at 10 a.m. on July 19.

Wang said Chinese officials apparently hoped that the kidnaping would discourage other potential defectors, particularly since the Chinese Olympic team was about to arrive in the United States for the Summer Games in Los Angeles. News of Zhang's return, under usual Chinese procedures, would have been immediately transmitted to all delegations visiting the United States.

Wang said he was particularly worried by China's failure to call a news conference announcing Zhang's return, as was done in 1982 when another defector returned from Canada.

Chinese students at Boston University and the University of Rochester were abruptly returned to China last year, Wang said. One had begun dating a Taiwanese woman and another was accused of impregnating a fellow Chinese exchange student. Neither man had requested asylum, Wang said, and the latter student was allowed to return and resume his studies several months later.