(CNN) Lt. Col. Alexander Vindman, the National Security Council's top Ukraine expert, testified this week that he was told not to talk with anyone about the July 25 call between President Donald Trump and the Ukrainian President, according to a source familiar with the testimony.

Vindman testified that he was concerned about the July call between Trump and Ukrainian President Volodymyr Zelensky, and he took those concerns to National Security Council lawyer John Eisenberg, who told him not to discuss the call, the source said.

Eisenberg and NSC lawyers moved the transcript of the call onto a higher security system than what's typically used for presidential calls — a decision that's among the issues Democratic impeachment investigators are probing.

Politico first reported that Eisenberg told Vindman not to discuss the call.

Vindman was one two National Security Council officials to testify before the House Intelligence, Foreign Affairs and Oversight committees this week behind closed doors. Vindman told lawmakers that while he was concerned about the July call, he testified that the rough transcript released by the White House was mostly accurate. But he proposed two edits to the transcript — to add references to Burisma, the company that hired Hunter Biden, and Vice President Joe Biden's tapes — which were ultimately not included.

Read More