WASHINGTON - He arrived in the United States from Ukraine as a child, became a US Army officer, served as a military liaison at embassies in Kiev and Moscow, and worked on the staff of the National Security Council in the White House.

That last role has put Lieutenant Colonel Alexander Vindman center stage in the impeachment inquiry being conducted by the House of Representatives into President Donald Trump for abuse of power.

Vindman testified before the House Intelligence Committee on Tuesday wearing his dress blue uniform displaying his combat infantry badge, campaign ribbons and a Purple Heart received for wounds suffered by a roadside bomb in Iraq.

The bespectacled 44-year-old career army officer with close-cropped black hair opened his testimony with a reference to his immigrant roots.

He praised his father's "courageous decision" to leave Ukraine for the United States to provide a better life for his three young sons after their mother died.

"I'm grateful to my father for his brave act of hope 40 years ago and for the privilege of being an American citizen and public servant," he said.

"As a young man I decided I wanted to spend my life serving this nation that gave my family refuge from authoritarian oppression.

"All three of us have served or are currently serving in the military," Vindman said in a reference to his two brothers, one of whom -- his twin brother -- also serves on the staff of the NSC.

A fluent speaker of Ukrainian, Russian and a "little English," he joked, Vindman earned a master's degree from Harvard University and was serving as the director for European Affairs at the NSC at the time of Trump's telephone call with Ukrainian President Volodymr Zelensky.

During the call -- which Vindman listened in on from the White House situation room -- Trump asked Zelensky to open an investigation into his potential 2020 Democratic presidential rival Joe Biden and his son Hunter, who served on the board of the Ukrainian gas company Burisma.

Vindman reported the call to lawyers at the NSC and told the House committee he felt that Trump's request to Zelensky was "inappropriate" and "improper" and a "partisan play."

"Frankly I couldn't believe what I was hearing," Vindman said. "There was an element of shock."

During the hearing, Republican lawmakers sought to damage Vindman's credibility by raising doubts about his loyalty because of his links to Ukraine.

Accused by Trump of being a "Never Trumper," Vindman stressed that he did not identify as a Republican or a Democrat and -- without referencing Trump specifically -- said that personal attacks on him and other witnesses were "reprehensible."

"I'd call myself never partisan," said the army officer, who maintained a calm and disciplined demeanor throughout his four-and-a-half hours of grueling testimony.

"We do not serve any political party we serve the nation," he said. -AFP