United States diplomats were jolted this spring when newspapers in Nigeria suddenly charged that the American ambassador had ordered the killing of two local political leaders.
The news headlines were backed up by a document that was said to be an internal US Embassy memo. The document said:
''Chief Abiola (a target) has outlived his usefulness to our service. . . . His flirtation with the opposition led by Obafemi Awolowo exemplifies the need to go ahead with operations Heartburn and Headache to solve the problem of these two. . . . The (State) Department must be well briefed on these wet affairs.''
Except for a single mistake, the document, which was forged, might have done serious damage to US-Nigerian relations. But US officials were able to show that it was a fake. The proof was in the wording.
The text used the term ''wet,'' which is a direct translation of a Russian espionage term for ''assassination.''
''It doesn't mean anything in English,'' a State Department official notes. ''So people just laugh. . . . Somebody (on the Soviet side) just slipped up.''
Even so, the Nigerian episode was troublesome. The reports initially gained some credence in Lagos, and were widely circulated in other African countries. The forgery had served its purpose.
More and more such ''dirty tricks'' are turning up around the globe these days. In the past year, the number of forged documents, faked news stories, and bogus telegrams has doubled. Most are believed to originate with Soviet intelligence, or its sister services in the East bloc.
A high-ranking State Department official concedes that Washington is concerned. The Soviet-backed campaign has a particularly important impact on the third world, where the press may be less questioning.
''Once you get this thing on the front page of the newspaper,'' the US official says, ''you can deny it, but you never really quite catch up with it.''
The finger of blame is pointed at the Soviet KGB's program of aktivnye meropriyatiya, or ''active measures.'' A key part of that program is the spreading of ''disinformation,'' or false stories that serve Soviet purposes.
A ''classic'' disinformation effort hit the headlines this week, US officials say. Soviet sources wove together a tale of satellites, espionage agents, and spy flights in their latest effort to explain the downing of Korean Air Lines Flight 7. The Soviets claimed:
* KAL 7 left Anchorage, Alaska, 40 minutes late, a delay that was planned so its flight pattern would match at crucial times with an American Ferret-D ''radio technical'' intelligence satellite passing over Soviet territory.
* KAL 7 carried 11 intelligence specialists aboard.
* Five US aircraft were in the vicinity when Flight 7 was there, in addition to the US frigate Badger.
* Coordination of all these efforts helped the US monitor a doubling of Soviet radio and radio-technical air defense activity as Flight 7 flew over Soviet territory.
''That's disinformation on the grossest scale,'' says a US official who studies Soviet methods. ''It's got all the elements: There are lots of facts in the story - the rotating of the earth, the passage of the satellite, and so forth. All happenstance.
''Then you embroider it a little bit. You say that the RC-135, which was on its regular mission, was on 'the mission,' and so forth. So that a person who is not an expert can say, 'That is possible.'
''What probably happened is that the boys in the KGB were tossed this problem a couple weeks ago and were told: 'Come up with something.' And this is what they worked out.''
Today, Soviet ''active measures'' have several major anti-US goals around the world, officials say. These include:
In Europe, to undercut support for deployment of US nuclear missiles. That is priority No. 1. Also, to blame the US for events in Poland, and to move blame for the assassination attempt on the Pope from the USSR to the US.
In Africa, to spread the story that the US is interfering in the internal affairs of black-led nations. At the same time, to link the US politically to white-run South Africa.
In Latin America, to undercut US support for El Salvador, and to tie the US to Britain on the Falklands issue.
In South Asia, to portray US policy as secretly favoring the Balkanization of the region.
In the Far East, to picture the US as guilty of chemical and biological warfare (thereby countering charges of yellow rain against the Soviets in Afghanistan and elsewhere). One method: to blame the US for recent outbreaks of dengue fever in Cuba.
US intelligence sources say the Soviets' active-measures program has mushroomed in the past few years. The office in charge has been changed from a department to a service, a higher ranking. Its budget is now believed to be between $3 billion and $4 billion a year.
Says a senior Central Intelligence Agency official: ''The forgeries we find are becoming more and more professional, and by using real documents and just manipulating a little bit, there is a great appearance of authenticity given to the documents.''
How serious is it? One expert says:
''It's important neither to overstate nor understate the impact. We do feel it needs to be realistically recognized that the Soviets, as a standard tool in the way they conduct their business, carry out these deception activities on a large scale.''
The Soviet purpose, he says, is to ''influence . . . public opinion.''
The best US response, he suggests, is to ''make available . . . information as it becomes available to us. Our feeling is that lots of sunshine and information is the best response.''
Even the US and European news media, however, aren't immune from a little Soviet manipulation now and then, officials say.
An example: press treatment of Soviet leader Yuri Andropov when he succeeded Leonid Brezhnev.
One concern in the Soviet Union at the time was that world opinion would not accept the former head of the KGB as leader of one of the superpowers. Would he be eyed suspiciously around the world?
Their solution: Portray Mr. Andropov as a ''closet liberal.'' Float stories that he was really Western-oriented, and thus more acceptable - that he reads Western novels, speaks English, drinks Western liquor, and enjoys Western jazz. There was even a report that he had dissidents around to help him keep in touch with trends in the USSR.
Most of this, even today, is ''unconfirmed,'' US officials say. But such details were carried in some news reports as fact, and Andropov was helped through the transition period.
In recent months, as the pace of Soviet disinformation efforts has grown, US officials have kept a list of the major ones. A few of them this year include:
Jan. 25, 26, 28. Patriot, a pro-communist New Delhi daily, published an ''expose'' of US policy in the third world. It charged the US is plotting to chop India into small states. For proof, it cited a ''speech'' by the American ambassador to the United Nations, Jeane Kirkpatrick, before the American Conservative Political Action Conference. In fact, Ambassador Kirkpatrick attended the conference, but gave no speech.
Feb. 7. Tiempo, a Spanish newsweekly, published extracts from a forged National Security Council memo, supposedly written by former national security adviser Zbigniew Brzezinski. The ''memo'' proposed actions to destabilize Poland. Three months later, the magazine published a denial by Mr. Brzezinski.
March 31. People's Daily Graphic, a government-owned paper in Ghana, reported that the US Embassy was working to overthrow the government. It relied on a West German report as evidence. Bonn denounced the report as false. This was later accepted by the Ghanaian government, but only after a period of great tension with the US.