The White House slammed top Ukraine official Lt Col Alexander Vindnam after he testified to an impeachment hearing that a phone call made by the US President Donald Trump to his Ukranian counterpart Volodymyr Zelensky was “improper” and had left him in “shock”.

Army Lieutenant-Colonel Alexander Vindman, a White House national security official for Ukraine, and Jennifer Williams, a State Department foreign policy aide to Vice President Mike Pence, appeared publicly for the first time.

“I was concerned by the call. What I heard was improper. And I reported my concerns” to White House lawyers, Vindman told the impeachment inquiry.

The hearings are investigating whether Trump abused his presidential power.

Lt Col Vindman, a decorated Iraq war veteran who serves in a senior role on the US National Security Council (NSC) testified before the House in his Army uniform.

Taking to Twitter, the White House tweeted, “Vindman made it clear that he gave information about President Trump’s phone call with President Zelensky to an unnamed individual. Schiff shut down questioning about the unnamed individual to “protect the whistleblower”. But neither one knows who the whistleblower is?”.

In another tweet, the White House said, “…we have Vindman, who never met or spoke to POTUS, testifying of alarming ‘demands’ in the 7/25 call.”

Last week, President Trump described the impeachment probe against him as “witch hunt”, saying he was “too busy” to watch it.

In October, President Trump opposed impeachment enquiry, saying that there should be no public hearings during the House of Representatives’ impeachment enquiry against him, and directed White House Chief of Staff Mick Mulvaney not to appear before the committees investigating Trump’s phone call to Ukraine.

Trump administration had ordered officials not to participate in the House enquiry. But lawmakers have spent weeks hearing from current and former government witnesses, largely from the State Department, as one official after another has relayed his or her understanding of events.

After almost a month of calling for greater transparency in the enquiry, the White House changed its strategy this week by prohibiting several of its officials from even testifying behind closed doors before the lower house committees.

Late September, the impeachment inquiry, which House Speaker Nancy Pelosi initiated over a complaint by an anonymous whistleblower, is looking into White House’s alleged efforts to withhold military aid to have Ukraine investigate a Trump’s political rival, Joe Biden.

(With inputs from agency)