When Donald Sellar, Washington correspondent for Canada's largest newspaper chain, was called by the FBI in June, he was more than a little concerned.

Sellar knew that officials in the U.S. intelligence community were upset about his articles for the Southam Inc. chain on Pentagon weapons testing. But he said he had not expected to be questioned by two FBI agents, who asked him to identify the source of his documents.

"It became immediately evident that they were not just trying to track down leakers, they were investigating me," Sellar said.

The incident highlights the Reagan administration's determination to crack down on unauthorized leaks. Concern about that was underscored yesterday by disclosure that President Reagan warned federal employes Tuesday that they could be prosecuted for disclosing classified information.

The FBI interview of Sellar was approved by Attorney General William French Smith. It was followed by a newspaper report that the Justice Department was considering seeking an indictment of Sellar, a Canadian citizen, under a statute dealing with theft of government property. This prompted complaints from the Canadian Embassy and media.

Justice and FBI spokesmen would neither confirm nor deny that Sellar is or was under investigation. Justice spokesman Mark Sheehan said department guidelines require the attorney general to approve all questioning of reporters.

Sellar, 37, who has returned to Canada, caused a stir with a report in October about secret negotiations to allow U.S. testing of cruise missiles and other weapons in Canada.

The day after he filed the story by computer transmission over a telephone line, an intelligence source warned him that U.S. officials were upset and that "there was a witch hunt under way for my sources," Sellar said.

Sellar said he was disturbed even more that the source quoted at length from the article, even though it had not yet been published. Sellar later reported that the National Security Agency apparently had intercepted his transmission of the story.

When FBI agent Douglas Gregory requested an interview, Sellar said, Gregory noted that Sellar had a White House press pass. Sellar said he wondered whether his credentials were in jeopardy.

Sellar said he refused to tell the FBI his sources for several military stories. He said they showed him the cover sheet of a classified document called "Air Force 2000," a military planning paper about which he had written, and asked whether he had obtained a copy from a federal employe. Sellar said he told them he had not.

The agents then asked if he had met with any Soviets, Sellar said. He said he told them two reporters had invited him to lunch with a reporter for the Soviet newspaper Izvestia. "They the agents were either trying to send me a message or send a message to my sources," Sellar said.

"If this had happened in Canada to an American journalist," he said, "there would be a huge public outcry in the U.S."

"We had to register our concern on this," said Patrick Gossage, the Canadian Embassy's information minister. "We were very concerned about a Canadian national being investigated for an alleged possession of documents that also were in the hands of American reporters. Why pick on a Canadian when these things go on all the time?"