The Leningrad correspondent for United Press International has been the target of official harassment and another UPI man has been refused a visa for a tour of duty in the Soviet Union, an official of the news agency said.

H. L. Stevenson, UPI editor-in-chief, said in San Francisco that Leningrad correspondent Emil Sveilis and his wife were being subjected to harassment by Soviet authorities because of the reporter's contacts with Soviet dissidents.

Stevenson said no reason had been given by Soviet officials for their refusal to grant a visa to Gerald Nedler, whom the agency wants to send to Moscow.

In a related development, the Soviet Union warned Western nations in an article carried by the official news agency Tass, that any confrontation over human rights at the June conference in Belgrade to review implementation of the Helsinki agreement on rights would harm East-West relations.

One of the leading Soviet critics of the government's policy on human rights, Maj. Gen. Pyotor Grigorenko, has been hospitalized with prostate trouble, family friends said.

The Czechoslovak Communist Party newspaper printed an appeal to President Carter by a woman in that country for the return of her two children from the United States. The children had been taken to the United States by their father in 1963. He was later killed in an automobile accident and a court blocked the children's return to Czechoslovakia.