The US complained to the Soviet Union for more than a decade about microwave radiation directed at its embassy in Moscow, but kept concerns secret from embassy staff for nine years, according to newly declassified documents.

The reported microwave radiation came to be known as the “Moscow signal” and was the source of frequent complaints from Washington. US officials were unsure of either the purpose of the signal or the potential health effects of long-term exposure to low-level microwave radiation.

The declassified documents, obtained by the National Security Archive at George Washington University, provide a historical perspective on current anxiety about “Havana syndrome”, a cluster of mysterious neurological symptoms afflicting scores of US diplomats and spies, which the US believes may have been caused deliberately by some form of directed energy weapon.

The first reference to the Moscow signal was in a June 1967 state department memo recording a conversation between the then US secretary of state, Dean Rusk, and the Soviet foreign minister, Andrei Gromyko, in which Rusk raised the matter of the “electro-magnetic signal” aimed at the embassy in Moscow. Gromyko expressed scepticism about the claim, but Rusk insisted there was “no doubt whatever about it” and sketched a rough diagram to illustrate his point. Gromyko said he would “look into the matter” but no change in the level of radiation was detected.

Over the years that followed, the microwave signals multiplied and intensified.

President Gerald Ford wrote to the Soviet leader, Leonid Brezhnev, in December 1975: “These transmissions have created levels of radiation within the embassy which may, in the opinion of our medical authorities, represent a hazard to the health of the American families living and working in that building. Indeed, in one particular case, they may already have caused a serious health problem for one member of our embassy staff.”

Ford was almost certainly referring to the ambassador Walter Stoessel, who became ill with leukaemia at that time, and died of the disease a decade later.

In his reply to the president, Brezhnev insisted the electromagnetic field around the US embassy was “of industrial origin”.

Despite US fears about the health effects, embassy staff were not informed, apparently because of concerns the story would leak to the media and upset arms control negotiations with Moscow. Stoessel’s illness was kept secret.

In a 1975 conversation with the then Soviet ambassador in Washington, Anatoly Dobrynin, the US secretary of state, Henry Kissinger, asked for the signal to be turned off before he made a planned visit to Moscow or, he joked, “You could give me a radiation treatment”.

“We really are sitting on it here, but too many people know about it,” Kissinger told the ambassador. If it was discovered that the Nixon and Ford administrations had known about the problem and done nothing to stop it, he said, “we will catch hell”.

The embassy staff were finally informed in 1976. A state department telegram from February of that year said employees should be briefed in small groups but they should not pass on the details to their dependants. However, the telegram recommended that pregnant staff or family members be medically evacuated immediately for tests.

The Soviet leadership took no heed of the US complaints and it is unclear when the Moscow signal was turned off, if it ever existed. US experts were mystified over the purpose of the microwave radiation, with the two leading theories being that it was intended to neutralise electronic intelligence gathering by the embassy, or to activate listening devices built into the structure of the embassy.

When the previous embassy building was demolished in 1964, dozens of microphones had been found embedded in its walls.