Congress is determined to get access to Donald Trump's calls with Russian President Vladimir Putin and other world leaders, the U.S. House Intelligence Committee's chairman said on Sunday, citing concerns that the president may have jeopardized national security.

"I think the paramount need here is to protect the national security of the United States and see whether in the conversations with other world leaders – and in particular with Putin – that the president was also undermining our security in a way that he thought would personally benefit his campaign," Democrat Adam Schiff said in a NBC's news program. 

The Democratic-led House last week launched an official impeachment inquiry into Trump in the aftermath of a whistleblower complaint from an individual within the U.S. intelligence community that Trump solicited interference by Ukraine in the 2020 election for his own political benefit.

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The whistleblower's complaint cited a telephone call in which Trump asked Ukrainian President Volodymyr Zelenskiy to open an investigation of former Vice President Joe Biden, a leader among Democratic candidates seeking to challenge Trump in 2020, and his son Hunter. Hunter Biden sat on the board of a Ukrainian gas company, Burisma Holdings Limited.

On Sunday, Ukraine's former Prime Minister Mykola Azarov said his country must investigate the activities of Hunter Biden to establish whether his role in the company complied with the country's law.

"It's a fact (his directorship and fees) and not made up. It should be investigated so that the 'i's can be dotted and the 't's crossed," Azarov told Reuters.

But Ukrainian prosecutors said they are not investigating Hunter Biden, but are looking into the legality of Burisma's activities before Biden joined its board. Burisma, which denies any wrongdoing, has faced allegations of dodging taxes and of improperly securing licenses for gas deposits.

Trump, in a series of Twitter posts on Sunday evening, said he wanted to "meet" the whistleblower, who he called "my accuser," as well as "the person who illegally gave this information" to the whistleblower.

"Was this person SPYING on the U.S. President? Big Consequences!" wrote Trump, who added without providing evidence, "I want Schiff questioned at the highest level for Fraud & Treason." 

The CBS program "60 Minutes" reported that the whistleblower is under federal protection after receiving threats.

Trump's July 25 phone call came shortly after the United States froze nearly 400 million U.S. dollars in aid to Ukraine, prompting concern that the president was using the taxpayer money approved by Congress as leverage for his personal political gain.

The complaint said White House lawyers directed that an electronic summary of the call be moved from the place where such things are usually kept to a separate electronic system reserved for classified and especially sensitive material - a move Democrats have called part of a cover-up.

"If those conversations with Putin or with other world leaders are sequestered in that same electronic file that is meant for covert action, not meant for this, if there's an effort to hide those and cover those up, yes we're determined to find out," Schiff said.

Schiff did not say whether he plans to subpoena that information. The White House did not respond to a request for comment.

Meanwhile, Schiff said the intelligence committee had reached an agreement with the whistleblower to appear before the panel and he hoped the whistleblower would appear very soon.

The Ukraine scandal arose just months after Special Counsel Robert Mueller finished an investigation into Russian interference in the 2016 election that concluded that Moscow waged a campaign of hacking and propaganda to boost Trump's candidacy.

Mueller's investigative report, released in redacted form in April, laid out numerous contacts between Russian officials and Trump's campaign, but found insufficient evidence to determine that a criminal conspiracy had taken place.

Trump has defended his phone call with Zelensky and called the whistleblower a "political hack."

White House adviser Stephen Miller took up the attack on Sunday, accusing the whistleblower of being part of a "deep state" government conspiracy against Trump.

"I know the difference between a whistleblower and a 'deep state' operative. This is a 'deep state' operative pure and simple," Miller said.

"The president of the United States is the whistleblower. And this individual is a saboteur trying to undermine a democratically elected government," Miller added.

Trump's former homeland security adviser Tom Bossert, in a break with the president, called himself "deeply disturbed" by Trump's call with Zelensky, and not just because the president sought assistance in getting political dirt on Biden.

Bossert voiced concern that Trump also asked Zelensky to investigate the U.S. cybersecurity firm CrowdStrike, which looked into the hacking of Democratic Party emails in 2016 and helped lead American intelligence to conclude that Russia had done it, as well as the location of a Democratic server.

Bossert, who resigned from his post last year, said Trump was referring to "a debunked conspiracy theory that somehow Ukraine, not Russia, hacked the Democratic emails in 2016."

Schiff said Trump's personal lawyer Rudy Giuliani appeared to lay the foundation for Trump's call to Zelensky through his efforts to encourage Ukrainian authorities to investigate the Bidens.

Schiff told ABC his committee would decide whether to have Giuliani testify after the investigation fleshes out details of his involvement.

Giuliani said on Sunday he would testify only with Trump's approval.

Lawmakers are working out logistics to protect the whistleblowers identity and get security clearance for lawyers who represent the whistleblower. A person close to the whistleblower said on Sunday many issues remained to be worked out.