(Reuters) - U.S. Special Counsel Robert Mueller is focused on a meeting at the height of the 2016 presidential campaign between Donald Trump's then campaign chairman Paul Manafort and a business associate prosecutors say has ties to Russian intelligence, according to a court filing on Thursday.

The Aug. 2, 2016 meeting between Manafort and Konstantin Kilimnik was one of several at which the two discussed a topic related to Ukraine, according to the transcript of a sealed Feb. 4 court hearing in Manafort's criminal case in Washington.

Mueller contends that Manafort lied about the number of times he and Kilimnik discussed the topic, the description of which is mostly redacted in the transcript.

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Judging by previous court filings the topic at issue appears to be a "Ukrainian peace plan" that was reportedly aimed at lifting U.S. sanctions on Russia - long an important objective of Russian President Vladimir Putin's government.

Andrew Weissmann, one of the prosecutors on Mueller's team, told the judge that Manafort's alleged lies about his meetings with Kilimnik were significant because they related to "what we think is going on, and what we think the motive here is."

"This goes, I think, very much to the heart of what the Special Counsel's Office is investigating," Weissmann said, according to the court transcript.

Mueller is investigating U.S. allegations that Russia meddled in the election and whether members of the Trump campaign coordinated with Moscow officials. So far, the investigation has ensnared 34 people. Moscow denies interference and Trump dismisses the probe as a political witch hunt.

Kilimnik, who was indicted last year on charges of witness tampering in Manafort's case, has denied having ties to Russian spy agencies.

In addition to the Aug. 2, 2016 meeting, which took place at the Grand Havana Club, a cigar bar in Manhattan, Manafort and Kilimnik also discussed the Ukraine-related issue in December 2016 and twice in early 2017, according to the transcript.

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One of the discussions took place in person in January 2017 when Kilimnik was in Washington for Trump's inauguration, the filing said.

The closed hearing was held so the judge could weigh evidence supporting allegations by Mueller's office that Manafort had lied to prosecutors about five subject matters in breach of a plea agreement struck in September.

Manafort, who was convicted of financial crimes by a jury in a separate case in Virginia in August, has said any inconsistencies in his statements to prosecutors and the grand jury were not intentional.

The transcript also showed that Manafort continued to work for a client in Ukraine in 2018 even after he was indicted by Mueller.