Sen. John McCain on Tuesday threatened to block President Trump's deputy defense secretary nominee during a tense exchange over providing weapons to Ukraine.

McCain took issue with Boeing executive Pat Shanahan's written response to the Senate Armed Services Committee that he would need to take a look at equipping Ukrainian allies with lethal arms, something McCain has long advocated.

The chairman hammered Shanahan with pointed questions over his time at Boeing and whether he did his homework on the Ukraine question during the opening minutes of what was expected to be a smooth confirmation hearing.

.@SenJohnMcCain: "Do not do that again Mr. Shanahan or I will not take your name up for a vote before this committee. Am I perfectly clear?" pic.twitter.com/JNpIBkb9yB— CSPAN (@cspan) June 20, 2017

"Have you not been aware of the issue? You're not aware of the actions of the Senate Armed Services cCommittee? Have you not been aware of the thousands of people who have been killed by Vladimir Putin? Have you missed all that in your duties at one of the major defense corporations in this country?"

Shanahan, a vice president who helped improve Boeing's troubled 787 Dreamliner program, changed his earlier submitted answer under McCain's withering questioning.

"I support equipping the Ukrainians," he told the Arizona Republican.

McCain continued with the line of questioning, still not satisfied.

"Not a good beginning, not a good beginning," McCain said. "Do not do that again, Mr. Shanahan, or I will not take your name up for a vote before this committee. Am I perfectly clear?"

Shanahan responded by simply saying "very clear."

The hearing continued on a rocky footing with McCain later claiming Shanahan was ducking questions from senators.

Shanahan would help fill a key position under Defense Secretary Jim Mattis at the Pentagon if he is confirmed.

But he must first pass a vote in the Armed Services Committee, then his nomination would be taken up for a final vote on the Senate floor.