Washington (CNN) President Donald Trump on Sunday escalated his rebuke of the anonymous whistleblower at the center of the mounting Ukraine controversy after House Democrats launched an impeachment inquiry against him, asserting that he deserves to "meet my accuser."

"Like every American, I deserve to meet my accuser, especially when this accuser, the so-called 'Whistleblower,' represented a perfect conversation with a foreign leader in a totally inaccurate and fraudulent way," Trump tweeted before taking aim at House Intelligence Committee Chairman Adam Schiff who confirmed earlier Sunday there is a tentative agreement for the whistleblower to testify before his committee.

"His lies were made in perhaps the most blatant and sinister manner ever seen in the great Chamber. He wrote down and read terrible things, then said it was from the mouth of the President of the United States. I want Schiff questioned at the highest level for Fraud & Treason," Trump said.

Lawyers for the whistleblower sent a letter to the acting Director of National Intelligence Joseph Maguire on Saturday outlining "serious" safety concerns for their client as Trump continues to take aim at the person.

"The purpose of this letter is to formally notify you of serious concerns we have regarding our client's personal safety," the letter says, adding that recent comments by Trump are reason for "heightened" concern.