West German Chancellor Helmut Kohl refused to allow close surveillance of a secretary in his office and her husband only three weeks before the couple defected to East Berlin, a senior government official said today.

Hans Neusel, the second-ranking official in the Interior Ministry, told a press conference today that Kohl was informed on Aug. 28 that the couple was suspected of espionage but decided against putting them under full-time observation because the evidence was not conclusive.

The Bonn government announced yesterday that Herta-Astrid Willner, a secretary for Kohl's leading domestic affairs adviser, sent a letter from East Germany notifying the Chancellery that she was leaving her post and joining her husband in East Berlin.

Herbert Willner, a senior associate at a research institute financed by the the Free Democratic Party, a coalition partner in Kohl's government, also sent a letter to his employer saying he feared arrest on "a criminal offense against the security of the Federal Republic."

According to Neusel, Kohl believed that placing the couple under full scrutiny was "inopportune" even though Hans Joachim Tiedge, a leading West German counterintelligence officer, had fled to East Berlin the week before and could have informed the communist authorities about any agents under investigation in the West.

Heribert Hellenbroich, the former intelligence director, recently was forced into early retirement because he failed to remove Tiedge as a security risk even though he knew for some time of the counterspy's severe drinking and debt problems.

Neusel, who attended the Aug. 28 meeting along with Interior Minister Friedrich Zimmermann, admitted that he counseled Kohl that evidence was not hard enough to warrant wiretapping and round-the-clock surveillance. "Perhaps we made the wrong decision," he said.

The opposition Social Democrats intensified their demands today for Zimmermann to take political responsibility for the country's most serious spy scandal and resign from his post. Zimmermann has rebuffed earlier calls to step down and Kohl has backed the minister if only to thwart a right-wing rebellion within his fractious conservative coalition.

Tiedge was in charge of tracking East German spies and was intimately involved in the Willner case before his defection. Neusel said it was highly plausible that East Berlin, after interrogating Tiedge, could have tipped off the Willners and warned them to seek sanctuary.

The couple, who were vacationing in Spain at the time, also could have decided on their own to leave for East Germany after seeing a front-page headline in the mass circulation daily Bild hinting that authorities were hunting for a spy in the chancellor's office, Neusel said.

He acknowledged that the government is taking urgent action on two other espionage cases in the wake of the Willners' defection, but Bild reported today that at least five cases are now under urgent investigation, including one involving the Defense Ministry.

Herbert Willner first came under suspicion in 1973 when a western embassy reported that he was probing too insistently on defense matters. He then worked at the Free Democratic Party headquarters.

Counterintelligence officers last May sought to place Willner under a close watch, but their request was refused. Hellenbroich complained to Zimmermann in late June about the rejected wiretap request but was told he lacked sufficient grounds.