Washington (CNN) President Donald Trump on Tuesday dismissed the escalating impeachment inquiry on Capitol Hill during a jovial Rose Garden appearance, but sources say there is mounting frustration over the cavalcade of administration witnesses providing testimony.

"I wouldn't worry about it, fellas, I wouldn't worry about it," Trump joked alongside Stanley Cup champions the St. Louis Blues.

Inside the West Wing, sources say, there is escalating concern about administration witnesses who are giving depositions on Capitol Hill.

The frustration comes as the Office of the Vice President said it will not comply with House requests for documents related to the impeachment inquiry, according to a Tuesday letter from Matthew Morgan, counsel for Vice President Mike Pence.

The letter cited White House counsel's previous letter to House Speaker Nancy Pelosi questioning the legitimacy of the impeachment inquiry, saying the process "calls into question your commitment to fundamental fairness and due process rights."

Read More