When a little-known American film company arrived in Russia to make a TV documentary series on World War II, they got some unexpected rewards:

Exclusive filmed interviews with Soviet leader Leonid I. Brezhnev, Premier Alexei Kosygin and other powerful Communist Party figures.

An unprecedented, first-time-in-30-years opening of the Kremlin's vast film archive.

The services of more than 500 Soviet film archivists, technicians and editors to cull through millions of feet of war footage.

And a private plane to carry a film crew and series narrator Burt Lancaster around the country without delays.

Titled "The Unknown War," the series of 20 one-hour programs is to begin in the fall of 1978, marketed by the show's producers, a New York-based media sales company called Air Time Inc., whose owners want to get into major television production and believe "The Unknown War" will do it for them.

To produce it, the firm sought out the best-known authorities on the Soviet Union in World War II, and on film documentaries. It was natural selection: Harrison E. Salisbury for the history, Isaac Kleinerman to oversee the documentary treatment.

Salisbury, now retired, was The New York Times Moscow correspondent here during part of the war, a Pulitzer Prize-winning journalist and authority on the Soviet Union.

Kleinerman has a string of credits that starts at the legendary beginning of television documentary series, "Victory at Sea," and has continued unbroken for 25 years. At 61, Kleinerman had started a new company as an independent documentary producer after 19 years with CBS in which he produced the Emmy-award winning series "20th Century" and its successor "21st Century," as well as dozens of other films.

Salisbury laid down the general outlines of the 20 segments, and they were brought to the Soviet by Fred Weiner, 35-year-old vice president of Air Time who had dreamed up the idea of the television series after a 1976 visit here that impressed him with his own ignorance of the sacrifice and heroism of the Russians in the war.

THe Salisbury outline was reviewed, as Weiner says, "at some very high levels in the government" and there turned out to be deep interest within the Soviet leadership in the success of the American venture. He got the go-ahead, and the project was turned over to the central film documentary studio in Moscow, repository of literally millions of feet of Soviet news and propaganda films.

Enter K Roman Karmen, a 71-year-old Soviet cinematographer who has manned his camera, he boasts, in four wars over the past 41 years (Spanish Civil War, Sino-Japanese War, World War II, Vietminh-French War). In his own right a kind of Soviet version of Kleinerman, Karmen was the subject of a retrospective of his 60-plus films at the museum of modern art in New York in 1973.

"When we looked at Salisbury's work, we were deeply moved with the friendly spirit of it," said Karman. "We saw they had sincere wishes for the truth . . . they wanted to tell Americans about the sacrifices of the Soviet Union during the war."

Kleinerman was hired as adviser and consultant to oversee the actual selection of war footage and the editing to match the general outline of each segment. Burt Lancaster, one of the few veteran Hollywood stars left who has appeared only infrequently on television, was landed by Weiner to narrate the series.

His project had hit big-time dimensions.

And then the work began.

As the teams of archivists under Karmen's direction began probing the vast libraries of their central film studio here, they made an astonishing discovery: a mother lode of perhaps 3 million feet of war footage that had never been seen by the public. The films were the "outtakes" from newsreels made throughout the war.

"They were in such a hurry to get the stuff into newsreels that they simply cut out what they wanted and threw the rest into storage," Kleinerman said, "No one's really seen it before."

"It was like calling a spirit out of a bottle and not getting it back, says Karmen, describing the delight and then dismay of his crews as they began to sift through the long-neglected footage.

Because of this and other problems, the project has gotten somewhat behind schedule. But Weiner and Kleinerman, who have been here a total of eight times since the original proposal was presented to the Soviets last year, feel it's generally on track. Two segments are within a few weeks of being finished and most of the others are in reasonable shape, according to Kleinerman.

Under the current plan, the two segments will be flown to New York next month where the final script under the supervision of Salisbury, Kleinerman, Weiner and others will be settled, narrated by Lancaster and the sound mixed onto the film.

Most of the Soviet war footage was shot by a special, 250-member film unit that operated throughout the conflict on all its fronts, from the Baltic to the Balkans and eastward to Manchuria.

Kleinerman, who has spent perhaps half his professional life dealing with archive war footage, says "No one got closer to the front-line action than these photographers. Some of the sequences are positively unbelievable. In some cases, they went beyond their own lines to film things."

According to Karmen, who was a combat moviemaker during the war, 50 of the 250 photographers perished in service.

Adhering to Salisbury's outline, Karmen's staff has reviewed and edited much of this footage into programs that will deal with the war as it unfolded, beginning June 22, 1941 when Hitler's armies exploded across the Soviet frontier, and ending more than four years later with the Red Army in shattered Berlin.

Among the episodes are "The Seige of Leningrad," "The Battle for Moscow," "The Battle for Stalingrad," (two segments for this epic struggle), and a final segment as a reprise and memorial to the vanished victims. Karmen directed the first program, "June 22," and the list, "The Unknown Soldier."

(Air Time has been in touch with station groups owned by NBC, Westinghouse and Metromedia, according to Weiner, but no offers will be made until one or two completed episodes are seen in February. "The real thrust of our sales effort will be in March," said Weiner, when station managers gather for the National Association of Television Program Executives convention.)

One of the major highlights of the series, say the producers, will be an interview with Soviet leader Brezhnev, in his Kremlin office with his great granddaughter of his knee. The interview was filmed Oct. 28 and Karmen proudly showed a reporter some color stills from the session showing a smiling, relaxed Brezhnev.

In addition to the war footage, the series includes some locations with Lancaster. When it seemed they might not be able to keep to their shooting schedule (Lancaster spent 10 days in Russia), the company found itself with its own twin-engine private plane to make sure the scenes got shot in time. "They do it just like us," Kleinerman says, especially when the Kremlin is watching.

But desptie all the cooperation - of which there has been an enormous amount - there have been inevitable strains.

'At first, I was unhappy with the title, "The Unknown War," says Karmen, "Because this war was known so well by us. But now I am happy, this is a clever thing, a little ironic, but aimed at the postwar generations who know nothing of the war." He takes a drag on an American filter and lounges back a little farther. "I was talking recently wiht some young Americans and asked them what they knew of the war. They asked me if that was the time the Americans and British fought the Russians!"

To Karmen, who sports on his left lapel the red and gold star of a Hero of Socialist award, this is akin to sacrilege.

To Kleinerman, who has been the archieve films of Germany, Japan, Britain and American among the major wartime powers, the young Americans' reaction is a bizarre aberration. "Look," he growls, "this is a country where 20 million people lost their lives in World War II. This is incomprehensible to the American people." He shakes his head in bewilderment, listening to Karmen's anecdote.

To Weiner, who thought up the idea, hired the best people, and convinced the Russian to do what they had never done before - and whose company has about $5 million riding on the outcome - the Karmen anecdote lies at the heart of his inspiration to fill in the gaps.