New York: A US national security official was so alarmed by US President Donald Trump's behaviour on his infamous phone call with the Ukrainian President that he believed it was his "duty" to quickly report it to a White House lawyer.

Lieutenant Colonel Alexander Vindman, a decorated military veteran responsible for coordinating US policy on Ukraine, said he reported the call because he feared Ukraine was being drawn into a "partisan play" to benefit Trump politically.

Vindman and State Department official Jennifer Williams appeared before the House Intelligence Committee on Tuesday (Wednesday AEDT) for the impeachment inquiry into Trump.

They were the first witnesses to publicly testify about first-hand knowledge of the July 25 phone call between Trump and Volodymyr Zelensky.