Yes. Here's your answer: Brian Keith was an American, as were both his parents. None of his family on either side knew Russian, or had any connection to Russia or Russians, so it is safe to presume that he could not have been a fluent Russian speaker.
The sentence in IMDbs Trivia section that said that Brian Keith could speak fluent Russian in real life has now been corrected.
Brian Keith faking it as Meteor Russian

• Daily News, Kentucky, November 13, 1979 Copley News Service
  HOLLYWOOD Even though Brian Keith really doesnt speak Russian, youd never guess it when you hear him rattling away in Meteor.
  Unless youre Russian, Natalie Wood points out.
  Keiths role in the big AIP movie about impending collision between a five-mile-wide meteor and Earth is that of a Russian scientist who speaks only Russian throughout the film, and, considering the circumstances, he does it admirably.
  But Natalie, whos of Russian extraction and learned Russian when she was learning English, does it better, and, from her expertise, noted flaws in Keiths pronunciation.
  He was wonderful, says Natalie, one of Keiths co-stars in the picture, but he spoke with an American accent. However, only a Russian would notice that.
  That Keith could deliver his lines in any kind of Russian, American-accented or otherwise, was amazing, because he doesnt know the language and had to learn his role phonetically.
  Nevertheless, he not only managed to say what had to be said, but did it with appealing warmth and humor.
  His performance is particularly remarkable since he was a latecomer to the cast, called in when Donald Pleasance, whod been set, was trapped in another production which ran longer than scheduled.
  We got lucky with Brian, says satisfied Samuel Arkoff, AIP president. His innuendos were wonderful.
  Having accepted the role, Keith was turned over to a Russian teacher who polished his pronunciation.

Natalie Wood didn't surprise me. Her parents are Russian. But, I had no idea that Brian Keith could speak Russian. Wow. He did a great job.
I miss both of these actors who died so tragically.

According to his bio, he was fluent in Russian, as was Natalie Wood.

I'm wondering the same. He must have known Russian.

Ok, so Natalie Wood spoke a few words of Russian. What about Brian Keth's lenghty monlogues in Russian? No actor could memorize and pronounce so much of a difficult language unless he was a native speaker. Does anyone know about this?

meinari, I suspect that Keith really could speak SOME Russian at least. I can't say that I'm fluent myself but have a Russian background and he sounds very authentic to me. He played a similar role in the 1982 mini-series "World War III". Have also read he spoke Russian with Alan Arkin off-screen while they were filming "The Russians Are Coming! The Russians Are Coming!" The statement that he was fluent in Russian is in the IMDb trivia section and has not been "corrected" as you said.

Here's the answer: Brian Keith was an American, as were both his parents. None of his family on either side knew Russian, or had any connection to Russia or Russians, so it is safe to presume that he could not have been a fluent Russian speaker.
The sentence in IMDbs Trivia section that said that Brian Keith could speak fluent Russian in real life has now been corrected.
Brian Keith faking it as Meteor Russian

• Daily News, Kentucky, November 13, 1979 Copley News Service
  HOLLYWOOD Even though Brian Keith really doesnt speak Russian, youd never guess it when you hear him rattling away in Meteor.
  Unless youre Russian, Natalie Wood points out.
  Keiths role in the big AIP movie about impending collision between a five-mile-wide meteor and Earth is that of a Russian scientist who speaks only Russian throughout the film, and, considering the circumstances, he does it admirably.
  But Natalie, whos of Russian extraction and learned Russian when she was learning English, does it better, and, from her expertise, noted flaws in Keiths pronunciation.
  He was wonderful, says Natalie, one of Keiths co-stars in the picture, but he spoke with an American accent. However, only a Russian would notice that.
  That Keith could deliver his lines in any kind of Russian, American-accented or otherwise, was amazing, because he doesnt know the language and had to learn his role phonetically.
  Nevertheless, he not only managed to say what had to be said, but did it with appealing warmth and humor.
  His performance is particularly remarkable since he was a latecomer to the cast, called in when Donald Pleasance, whod been set, was trapped in another production which ran longer than scheduled.
  We got lucky with Brian, says satisfied Samuel Arkoff, AIP president. His innuendos were wonderful.
  Having accepted the role, Keith was turned over to a Russian teacher who polished his pronunciation.

I'm glad someone else took note of that. His Russian was very convincing in this film.
Ignoring: QuesterJonesV, MythicCDXX, Creeping Jesus/Judas, RonPaul_Lies, Digby (and aliases), ibestupid, Holiday_Hobo, sharon_18, TilaMoo, Okie-from-Muskogee/boo321, NorCalNik, Nullifidian

From the description floating around about his parents, they don't appear to be Russian immigrants. Also says that he spoke fluent Russian - but that's hard to verify. His wife set up a webiste - http://www.briankeith.com/ - but unfortunately doesn't mention anything about his Russian.

Natalie Wood was fluent in Russian. She could have helped coach him.

Brian Keith was always one of my favorite actors. The way he died was truly sad. His parents were American. Keith could have learned just enough Russian for the movie.