WASHINGTON (AP) — A career Army officer testified Tuesday that President Donald Trump's call with Ukraine was "improper," as Republicans tried to undercut the national security official with pointed exchanges questioning his loyalty to the U.S. during a remarkable day in the impeachment hearings.

Arriving on Capitol Hill in military blue with medals across his chest, Lt. Col. Alexander Vindman told lawmakers it was his "duty" to report his concerns about the call. But he deflected repeated Republican efforts to divulge everyone he told about it -- thwarting Trump allies' attempts to identify the anonymous whistleblower who spurred the impeachment probe.

Vindman, a 20-year military officer who received a Purple Heart for being wounded in the Iraq War, was among the officials who listened in to the July 25 call when Trump asked Ukrainian President Volodymyr Zelenskiy for a "favor" -- investigations of Democrat Joe Biden and other issues.

"Without hesitation, I knew I had to report this," Vindman told the House Intelligence Committee. "It was inappropriate, it was improper for the president to demand an investigation into a political opponent."

The testimony launched a pivotal week as the House's historic impeachment investigation reaches further into Trump's White House. Democrats say Trump's pressure on Ukraine to investigate former Vice President Joe Biden while withholding U.S. military aid to Kyiv may be grounds for removing the 45th president. Republicans have argued both that there was no linkage between the two matters and that there is nothing inappropriate even if there was.

Vindman, an official at the National Security Council, testified alongside Jennifer Williams, his counterpart at Vice President Mike Pence's office. Both said they had concerns as Trump spoke with the newly elected Ukrainian president about political investigations into Biden.

Trump insists Zelenskiy did not feel pressured and has cast the impeachment probe as a partisan affair aimed at pushing him from office.

It wasn't the first time Vindman was alarmed over the administration's push to have Ukraine investigate Democrats, he testified.

He highlighted a July 10 meeting at the White House when Ambassador Gordon Sondland told visiting Ukraine officials they would need to "deliver" before next steps — a meeting Zelenskiy wanted with Trump.

"Ambassador Sondland referred to investigations into the Bidens and Burisma in 2016," he testified.

On both occasions, Vindman said, he took his concerns about the shifting Ukraine policy to the lead counsel at the NSC, John Eisenberg. Republicans later criticized him for not reporting to his direct supervisor.

An immigrant who came to the U.S. as a toddler from Ukraine, Vindman opened his testimony by assuring his father he would be "fine for telling the truth."

Yet Vindman spent long stretches fielding Republican attacks on his loyalty to the U.S. and his career in public service. The Republicans' lead counsel asked at one point about an offer to Vindman from a Ukrainian official to become the country's defense minister.

Vindman called it "comical" and said he swiftly reported it up his chain of command.

"I'm an American," Vindman said. "And I immediately dismissed these offers."

Later Tuesday, the House committee was hearing from former NSC official Timothy Morrison and Kurt Volker, the former Ukraine special envoy. On Wednesday, Sondland, the U.S. ambassador to the European Union, is to appear. Much of Sondland's private testimony to lawmakers has been contradicted by other witnesses.

At the White House, Trump said he had watched part of the day's testimony and slammed the ongoing impeachment hearings as a "disgrace." Over the weekend, Trump assailed Williams as part of the "Never Trumpers" who oppose his presidency, though there is no indication she has shown any partisanship. Trump allies have also repeatedly attacked Vindman's loyalty.

Vindman acknowledged the attacks during his testimony and appeared prepared to defend his loyalty to the United States. When the top Republican on the committee, Rep. Devin Nunes, addressed him as "Mr. Vindman," the colonel reminded him to address him by his rank.

Republicans tried to prompt the witnesses to name the still-anonymous whistleblower.

Nunes asked them who else they talked to about their concerns, bearing down once Vindman acknowledged one was from the intelligence community. The whistleblower is a CIA official, according to people with knowledge of the matter.

Vindman said he does not know who the whistleblower is. He has previously said it is not him.

Trump ally Jim Jordan pressed Vindman if he ever leaked information. "Never did, never would," Vindman testified.

Vindman is being provided security by the U.S. Army and local law enforcement, according to a U.S. official. The official said the Army is prepared to take additional steps, if needed, including moving Vindman and his family to a more secure location on a base.

Williams, a career State Department official who has worked for three presidential administrations and counts former Secretary of State Condoleezza Rice a "personal hero," said the Trump phone call was the first time she had heard anyone specifically seeking investigations from Ukraine.

The reference to Biden and his son Hunter "struck me as political in nature."

Williams testified the Trump phone call was unlike about a dozen others she had heard from presidents over her career. When the White House produced a rough transcript later that day, she put it in Vice President Pence's briefing materials. "I just don't know if he read it," Williams testified earlier in her closed-door House interview.

Pence's role throughout the impeachment inquiry has been unclear.

Vindman said Trump's remarks strayed from the talking points prepared for him. And both witnesses noted the use of the word "Burisma" on the call. That was a reference to the gas company in Ukraine where Hunter Biden served on the board.

They both said Zelenskiy had mentioned "Burisma" on the call, but testified it was missing from the rough transcript released by the White House.

At the time of the call, the officials were just beginning to make the link with the stalled military aid — $391 million approved by Congress— that Ukraine was relying on as it confronts neighboring Russia.

Vindman said the uneven power dynamic between the presidents of the East European ally and the U.S. made the demand obvious.

"The culture I come from, the military culture, when a senior asks you to do something ... it's not be taken as a request, it's to be taken as an order," he said.

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Associated Press writers Alan Fram, Zeke J. Miller, Laurie Kellman, Colleen Long, Eric Tucker, Lolita Baldor and Jill Colvin in Washington and Bruce Schreiner in Louisville, Kentucky, contributed to this report.